Dear spellcasters, 

What spells have you found that are the best to use against fiends? I am playing a campaign that features them heavily and id love to somehow get a spelllist that doesnt suck. Many if not all of them have magic resistance so im trying to avoid save or suck spells but a spell like "banishment" would be pretty good. 

So im looking for tactics for a spellcaster to fight against these types of creatures. Im playing a divine sorcerer so I have the sorcerer and the cleric spell list at my disposal.

Thanks!

Avoid cold, fire and poison. Also lightning. And non magical weapon attacks. 
Divine sorcerer, maybe focus on radiant and force damage when you can. buffing / healing allies. 

Hold Monster is a solid choice if your party has a few melee damage dealers.  A fighter with Action Surge, a Rogue with Sneak Attack, a Paladin with Smite, etc.

Though not prepared for most adventures, Protection from Evil and Good (1st lvl), Beacon of Hope (3rd lvl) and Dispel Evil and Good (5th lvl) are excellent in any campaign with numerous fiends.  Anything you can buff your allies with to prevent them from suffering fear, confusion, poison or charm spells/abilities.  

Absorb elements or Protection from Energy is useful if you know you are going to deal with Ice Devils or higher devils that use fire.  

Freedom of Movement is very good to maintain mobility since many devils have grapple abilities.  Bless is useful for improving saving throws and attack rolls more generally.  

I would also recommend See Invisibility since higher level devils often use Clairvoyance and imps to spy on the party.

Banishment

Spells that buff your allies, bypass saves with attack rolls, bypass saves with checks, and spells that don’t use a resisted damage type.

some damaging spells don’t use saves or attacks. Spiked growth and cloud of daggers can be used to great effect if the party works together.

spirit guardians is such an effective spell it may be worth using despite the magic resistance.

A spell like fog Cloud may be good too as it’s low level and neutralizes the advantage that devils gain by using magical darkness with their devil’s sight. 

holy weapon on a martial combatants weapon can get pretty ridiculous. Holy weapon can be cast on ranged weapons too.

Thunder is my go to type for things like chromatic orb as there's only a couple with resistance and even then resistance is no worse than the alternatives. Force, radiant and psychic make for better options, but are harder to get.

Aside from that, in the past we've had a fiend that didn't want to stay dead so rather than continuing to kill it or journeying to it's home plane we decided on a simpler solution.  We beat it unconscious, held a banishment then everyone attacks it then it gets banished.  Needs DM approval to work that way though, if you're doing it by RAW, ready the spell to go off after the first successful melee attack, dropping it in its home plane with two failed death saves.

Ya the magic resistance fights on top of creatures with high saving throws in general are tough for casters, thats where your martial classes with magic weapons really shine. You could cast Haste on your strongest party member to help them do more dmg, throw out Bless, then a few Guiding Bolts and Spiritual Weapon (depending on lvl, that would be for lower to mid tier), if you have 5th lvl spells a Summon Celestial will work nicely as they'll multi attack and do radiant dmg, Bigby's Hand is another 5th lvl spell that would do magical dmg since its dmg from a spell, etc. But ya at lower to mid levels before 5th lvl spells you're best to buff your allies and throw attack spells out for the most part i'd say.

Agree with the general trend here. Spells that buff the martials, spells with attack rolls, spells with ability checks like Bigby's Hand and Telekinesis all get around magic resistance.

Depends on level.  Higher level stuff really starts to do wonders against them.

Synaptic Static is a really good one to get off early, if a bit high level. If you can get three or more in it, it will change the battle.

Also, for the banishment technique, hit it with a spell that does damage over time,such as phantasmal killer, rather than everyone attack.

If you do it perfectly, you can send it to Abyss with 2 failed death saves AND a spell doing damage every round.

Most enemies don't get Death Saves. They exist more to give players a decent margin to be stabilized in after they hit 0 HP.

Here is what the rules have to say about monsters and death.

Most DMs have a monster die the instant it drops to 0 hit points, rather than having it fall unconscious and make death saving throws.

Mighty villains and special nonplayer characters are common exceptions; the DM might have them fall unconscious and follow the same rules as player characters.

Quote from Mog_Dracov >>

Also, for the banishment technique, hit it with a spell that does damage over time,such as phantasmal killer, rather than everyone attack.

If you do it perfectly, you can send it to Abyss with 2 failed death saves AND a spell doing damage every round.

 Depends on how your DM rules spell ranges on spell effects, particularly involving concentration.  In my experience, DMs are far less likely to object to you using nonlethal attacks on a fiend than they are to you maintaining concentration on a spell on a different plane of existence.

Basically spells that don’t require saving throws, as some folks stated above, Fiends have good saves and usually magic resistance or legendary saves. So indeed things like Bigby’s Hand, Telekinesis and even Animate Objects (although take care of non-magic damage) can make the difference.

My go-to spell to fight against creatures with high saves are the Summon spells from Tasha’s. I had a Necromancer that boosted Summon Undead thanks to his abilities and the Ghostly Spirit was quite effective.

Guiding bolt would be a good damage dealing spell. It does radiant damage and it gives your allies advantage against it on their next attack, this is great if there is a rogue on your team.

Banishment is good, but its only a tempoery fix. If its a high level fiend like a devil, they would just find they're way back to you, probably prepared for the fight.

Quote from 17ArtificerSpecialist >>

Banishment is good, but its only a tempoery fix. If its a high level fiend like a devil, they would just find they're way back to you, probably prepared for the fight.

 
Eh, lore is that it’s very hard for most fiends to get out of the Lower Planes on their own power; it’s part of the reason they entice mortals to summon and entreat them. And if you kill a fiend outside their home plane, they just get forcibly banished back and regenerate in any case. You do have a point that anyone who banishes them will be on their list, but ignoring special cases like Rakshasa, few outside of the very top tiers can pursue a direct vendetta from the Lower Planes.

Fiend challenges : Magic Resistance, Charm/Fear effects, Spellcasting, Devil's Sight + Magical Darkness, Immunity to fire & poison, Flight, Fire damage

Cantrips: Shocking Grasp, Chill Touch, Sapping Sting, Mind Sliver

Level 1: Bless, Guiding Bolt, Shield of Faith, Chromatic Orb, Healing Word, Shield, Absorb Elements, Magic Missile, Protect Good / Evil

Level 2: Spiritual Weapon, Alter Self, Calm Emotions, Invisibility, Earthen Grasp, Rime's Binding Ice (to freeze them out of the sky), See Invisibility, Shadow Blade

Level 3: Lightning Bolt, Spiritual Guardians, Counterspell (lots of fiends have some spellcasting), Dispel Magic, Daylight (lots of fiends have Devil's Sight and use magical darkness to their advantage), Fly, Haste, Summon Undead

Level 4: Black Tentacles, Fire Shield, Greater Invisibility, Storm Sphere, Summon Aberration (beholderkin), Vitriolic Sphere

Level 5: Telekinesis, Bigby's Hand, Steel Wind Strike, Summon Celestial, Wall of Force