So my question is simple, how do you protect those characters from spells like dispel magic?

In a campaign where I was playing a Bladesinger, I used counterspell to protect my buffs against dispel magic. In that build that was very effective. However, now I am DMing a campaign in which one of my players has the typical Hexblade that bases its tactics on Shadow of Moil, and the player is getting very frustrated since he can hardly use it (It is a game in which there are many clerics and, of course, there is always a magic dispel on hand that leaves him "naked"). Being a Warlock counterspell to defend against dispel magic is very expensive. And I would like to give him some "tip" so that he doesn't get so frustrated with his character. I have really tried to convince him to adapt his tactics to be able to work without Shadow of Moil, but I understand that he has invested a lot in that build.

In any case, beyond my game, I would like to know what tactics you use to defend your characters against dispel magic.

You don't, as a caster.

On the other hand, it is totally up to the DM to extensively use dispel magic. As a DM, I would just use the basic 3rd level dispel magic, ao against Shadow of Moil that would be a spell casting ability check for the enemy of the Hexblade to dispel it. That is typically something like a D20+3 or D20+4 against a DC 14 to dispel. That is 50-55% chance.

Fair game.

I don't want to intentionally play bad. I want to present a challenge to the players, not an easy ride. And most of them want it that way too. If not, they get bored.

What I also want is to be fair. And I think that's the key, setting difficult but fair challenges. And if a player, due to the interaction between the design of the game, and the design of his character, has a hard time, and doesn't know how to adapt, I don't want to limit myself either. What I do want is to teach him tactical options so he can get through the game if he can't do it on his own.

In this particular setting there is a cult specialized in hunting casters, which is one of the cores of the game. And of course, they're all going to go with dispel magic, and they're going to know how to use it to counter casters. I'm not going to have them conjure it to lvl3 when they're experts at it. It would not make sense and, in addition, it would be playing bad on purpose.

But in the end, yes. I think the only thing left for him is to rely on counterspell the few times he can cast it. Unless something else comes up. Or, what would be more advisable, to adapt his tactics.

Quote from Irrelevant_guy >>

Or, what would be more advisable, to adapt his tactics.

This is what should be happening. It's fine for players to have a go-to move, but they need backup plans for when that move isn't effective

If the character is aware that the party is going after a cult that specializes in hunting casters, then they should know relying on one particular concentration spell is going to be risky

Giving your NPC's weaknesses is not being a bad DM. Maybe they are novices and only have access to 3rd level spells, or don't have access at all. Maybe they had a run in with another target earlier in the day and no longer have spell slots. You're the DM, you control what happens in the world and if the PC's sometimes have runs of bad luck, NPC's might have them as well. 

Ways to defend against Dispel Magic.

1. Use a higher spell slot.  Dispel Magic has a harder time dispelling spells that were cast using a higher slot.
2. Counterspell their attempt to Dispel Magic. (as you mentioned)
3. Globe of Invulnerability prevents ALL Dispels Magic.  You can't even do it if you upcast the Dispel Magic, it simply will not work.
4. Kill/confuse/etc. their spell caster first.  

Quote from Mog_Dracov >>

Globe of Invulnerability prevents ALL Dispels Magic.  You can't even do it if you upcast the Dispel Magic, it simply will not work.

 Dispel magic can target magical effects, so the globe itself can be dispelled, though dispel cannot hit anything within the globe until it is removed (actually, no, globe only prevents targeting creatures or objects, and a spell is neither one, so shadows of moil can be targeted directly).

Quote from Pantagruel666 >>

Quote from Mog_Dracov >>

Globe of Invulnerability prevents ALL Dispels Magic.  You can't even do it if you upcast the Dispel Magic, it simply will not work.

 Dispel magic can target magical effects, so the globe itself can be dispelled, though dispel cannot hit anything within the globe until it is removed (actually, no, globe only prevents targeting creatures or objects, and a spell is neither one, so shadows of moil can be targeted directly).

 Sage Advice compendium disagrees: "Can dispel magic end globe of invulnerability? Yes, dispel magic can dispel the barrier created by globe of invulner­ability, but not any magical effects that are active inside the barrier."

However, I think the point is moot in this instance as the globe is immobile and would have to trap an opponent within. If said trapped opponent was the caster of dispel it also wouldn't protect from inside the globe. 

For these purposes I think Mog has the right approach, but I would reverse it and make 4 the primary approach.

Globe of invulnerability is something I've never thought to use to avoid dispel magic. But it is an interesting option. The big problem I see is Concentration. Generally what you want to protect from dispel magic are Concentration spells, so they can't be used together. But it is still one more option that had never occurred to me. Not for a warlock, of course.

Option 4 is one of the best tips to deal with this if you don't have counterspell. In the last game, myself and another player tried to explain to the warlock to wait to do his shadow of moil combo when the dispel magic threat is gone. We'll see in the next game how he handles it.

As offtopic thought, but motivated by some comments on the thread, I think that many players, especially players who have not played previous editions, are used to always winning. I have seen that many times. Groups of players who never consider running away, or not getting into fights that are clearly beyond their capabilities. Then the fight ends in TPK, and it makes them feel bad. But if they play with me, they have to understand that they are not always going to win and that many times there are other alternatives. Not every fight is going to be "clean house." Sometimes the way to win is to achieve a secondary objective (destroy an object, eliminate a specific objective, or something else). Sometimes it's better not to start a fight, use diplomacy, avoid detection, etc... I try to get my players to be analytical, use lateral thinking, and not play on autopilot. But some players have a hard time adjusting.

Quote from Irrelevant_guy >>

In a campaign where I was playing a Bladesinger, I used counterspell to protect my buffs against dispel magic. In that build that was very effective. However, now I am DMing a campaign in which one of my players has the typical Hexblade that bases its tactics on Shadow of Moil, and the player is getting very frustrated since he can hardly use it (It is a game in which there are many clerics and, of course, there is always a magic dispel on hand that leaves him "naked").

 The player's frustration is your fault. Nothing says you have to constantly cast dispel magic on him. If he is getting frustrated it means it is happening often. Let him use it and kick ass sometimes. 

There are other ways to attack such a character without removing the Shadow of Moil.

Another option to consider is use something other than Dispel Magic. Use Silence and Hold Person to stop the spell being cast in the first place. Change it up but still give the player opportunities to play his cool trick.

My go-to method of dispelling concentration spells on the party is massive damage or incapacitating conditions; you aren't picking on anyone when you fireball the party.

If they are getting frustrated at it, talk to the player separately and ask them what they think that their character might do about it, so that you can prepare for it.

For example, you might chat about it and they might say "I am going to try to find something to protect me against their dispel attempts", and you might say "There is a museum in the town which is featuring an exhibit on magical items, that might be worth a look", and you can add that in before the next session - now they're planning a heist to get a ring which, when attuned to, increases the spell level by 2 of any spells, for the purposes of dispel magic. 

Or they might look for ways to stop them from casting dispel magic, in which case you put a library in the town, where they can find some information on Spellblight Arrows, which can stop them from using their magic for a turn. Give them resources to make 3, and make sure that someone in the party has something that will use them, like a sharpshooter with a longbow or something. Tailor the thing they happen to find to the party, not the player - the sharpshooter might be happy to shoot the enemy cleric first so that the warlock can get in close with their magic still up.

It's a more satisfying thing to say "my character sees this happening and does this, which makes it happen less", compared with "My characer saw this happening, and asked god to change it, and god obliged". Maybe if they get a high enough cleric level for divine intevention, eh?

After prepping all the players with the forethought that "after this bit you will have some time in town, so let me know what you want to do", and perhaps guiding anyone who you might have some solutions for into seeking them, you can have a session or part-session in town where the players get to have a hand in what they are looking for or doing, which is often better when they have had time to think about it; otherwise you get the old "Um, I don't know, go to a tavern and get drunk?" response, which inevitably results in them getting less screentime, which then turns to them starting a bar fight so they can do something, which can spiral all the way to regicide if you let it.

Hope this helps!

Since you're the dungeon master, you're the one choosing to cast Dispel Magic at the warlock. You can choose to give him a break.

If you feel like continuing to attempt to dispel Shadow of Moil without being a jerk about it, however, you can always give your warlock a Wand of Counterspell. problem solved.

Quote from Irrelevant_guy>>

I have really tried to convince him to adapt his tactics …

That's a big one.

Personally I'm very fond of forcing players to come up with different tactics, and I struggle with one player of an archer, who if their archer can't fire a bow, does nothing. As a GM I'm going to give the player opportunities to feel like a badass, mowing down mooks at range, but I'm not going to make every combat solvable by one tactic. That's boring for me.

I understand that players get invested in their builds, but seriously, the adage "no plan survives contact with the enemy" has been around for a couple of thousand years. If you can't adapt, you die.

Quote from Irrelevant_guy >>

So my question is simple, how do you protect those characters from spells like dispel magic?

In a campaign where I was playing a Bladesinger, I used counterspell to protect my buffs against dispel magic. In that build that was very effective. However, now I am DMing a campaign in which one of my players has the typical Hexblade that bases its tactics on Shadow of Moil, and the player is getting very frustrated since he can hardly use it (It is a game in which there are many clerics and, of course, there is always a magic dispel on hand that leaves him "naked"). Being a Warlock counterspell to defend against dispel magic is very expensive. And I would like to give him some "tip" so that he doesn't get so frustrated with his character. I have really tried to convince him to adapt his tactics to be able to work without Shadow of Moil, but I understand that he has invested a lot in that build.

In any case, beyond my game, I would like to know what tactics you use to defend your characters against dispel magic.

 Remember, it is fundamental, core 5E design that PCs and NPCs do not share a physics engine.  Just as your NPCs are expected to have access to a wide swathe of abilities the PCs will never have, it's expected you will prevent your NPCs from having abilities that aren't fun.  In this case, you might have to cut off NPC access to Dispel Magic.  If you want to maintain your spellcasters as challenging opponents, homebrew them in a replacement spell that will be more fun for your table.

All good advice above. 

Was it clearly explained in session zero that there was this cult of caster hunters who would specialize in shutting down the players caster?  Did the warlock player just ignore that and now is mad? Or did they not realize it would become such an issue and is not having as much fun?

I ask because if they were not aware this would be such a significant issue to their build, then the problem stems from you not making that clear. If it was clearly explained and they just built there character that way anyway then the problem is of the players own making. 

Either way talk to them, help them figure out ways around it. Maybe throw less dispel magics at them, or save it for bigger fights. Or cast the dispel magic on other players, if applicable. Or allow them to possibly respec their spell choices or, worst case scenario, even class if it really will not be fun for the player. 

You don’t have to make the game easy mode, but you don’t have to constantly punish a player because of the choices they made building their character. 

A few questions:

How many spellcasters are in the party?  If the answer is more than just the warlock, why are the enemy clerics targeting the warlock for dispelling and not any of the other casters?

What percentage of encounters have casters capable of popping out Dispel Magic?  It honestly is going to come across as GM BS if every single encounter involves enemies who are tailor made to shut down a specific player's favorite tactic.  That tends to come across less as "you need to fight smarter" and more as "I'm punishing you for not playing the game the way I think you should."

Quote from 6thLyranGuard >>

A few questions:

How many spellcasters are in the party?  If the answer is more than just the warlock, why are the enemy clerics targeting the warlock for dispelling and not any of the other casters?

What percentage of encounters have casters capable of popping out Dispel Magic?  It honestly is going to come across as GM BS if every single encounter involves enemies who are tailor made to shut down a specific player's favorite tactic.  That tends to come across less as "you need to fight smarter" and more as "I'm punishing you for not playing the game the way I think you should."

He is the only full caster since before the campaign I warned them that magic was highly persecuted in this setting. Something I've copied from Midnight, nothing new here.

One of the pillars of the setting is that years ago the continent in which the game takes place was a "wizardcracy". The continent was divided into different states ruled by wizards and sorcerers (mainly), and people without magic were considered second class. Some states were more severe than others, but not being able to conjure back then was practically a sentence of slavery and servitude. That was until the arrival of a god (Nyxohr) who offered hope to the oppressed, causing great revolutions in the continent. After five centuries of wars and revolutions, the rebels won and now the casters are persecuted and hunted. Spearheading this pursuit are the priests of Nyxohr who, among other abilities, can detect the use of magic. Players begin the game being hired to bring a teenage sorceress whose powers have only recently begun to awaken, and whose parents feared for her life, to Dûrholm Forest. That forest, ruled by druids, and free from Nyxohr's influence, is the only refuge left for the casters.

The problem here is that every time the Warlock casts, there's a chance it will be detected (or reported by anyone who sees it, since people hate casters for obvious reasons). Plus, the mage hunters are after them from the beginning of the game. So they regularly face groups of mage hunters. And these groups always have at least one or two Nyxohr clerics with dispel magic.

I don't want to punish the player. But I also don't want to break the rules that I myself imposed on this setting. That is why I asked for strategies so that the player can deal with this, without feeling that I am artificially changing the game so that he can make his combo whenever he wants. That would be unfair to the other players and also kill the immersion. Ideally, he would adapt his tactics in order to deal with this situation. Either looking for ways to protect against dispel magic, or using alternative tactics.

If the priests have been hunting casters for so many years, using the same tactics of dispel magic, and the game is based in an area where the last refuge of the casters is hidden, then there is a good grounds for an eccentric inventor who needs help gathering artefacts to build a helmet/bracelet/belt/goggles/whatever which can ignore dispel magic.

Give them quests into ancient magical dungeons in the forgotten places, where they fight more traditional dungeon monsters rather than dispel-priests, and then reward them at the end with this magic item which renders the dispel-priests less dangerous!

This would also make it seem like foreshadowing of needing the item, so that they can face the priests on even ground.

Quote from ThriKreenWarrior >>

All good advice above. 

Was it clearly explained in session zero that there was this cult of caster hunters who would specialize in shutting down the players caster?  Did the warlock player just ignore that and now is mad? Or did they not realize it would become such an issue and is not having as much fun?

I ask because if they were not aware this would be such a significant issue to their build, then the problem stems from you not making that clear. If it was clearly explained and they just built there character that way anyway then the problem is of the players own making.

 Why would the DM give away the campaign plot in the session zero?

Anybody coming up with a character should be trying to be more than just a one-trick pony.