Remembering back to 100’s of comics tv shows and films where professor x was not attacked first ever but wolverine & colossus were....

It's all well and good to play with tactics and whatnot, but setting up encounters that the players don't like and then throwing up your hands and saying, "that's what my character(s) would do!" is a little disingenuous. To keep things interesting the party should encounter a variety of tactics and enemies that behave differently. Everyone just dogpiling on the weakest looking target isn't just not fun for the dogpiled guy, it's not really fun for anyone in the long run because every fight is the same. Ultimately you are on the hook for the situations you create, just like PCs are.

I try to threaten every PC in combat in the majority of encounters. Whether this means lurkers sneaking around to the back line or artillery trading shots with archers or brutes just blasting through the front line, I very rarely have everyone go after one guy because that's just not very interesting and it's not very fun. You can go on about superior tactics, but goblins haven't perfected their decision trees through 15 years of WoW raids. They are chaotic and emotional and savage.

D&D is not just a simulation, it's also a game. Playing enemies that do the tactically superior thing every single time might make for a great simulation, but that creates a very specific type of game that your players may not have bought in to.

I think the tactic being disputed presumes a sort of almost "fight card" knowledge level, where the opponents have weighed in and analysts have watched their last performance metrics.  How much reconnaissance has the enemy actually performed on the party who's just walked in on them?  Fog of war is real, and should work both ways, so the initial rounds I always play as both sides groping around to get a sense of each other as situational awareness grows.

Prepared attacks by opponents who've done their homework that gives them the tactical insight some DMs are presuming for their NPCs/Monsters shouldn't be that common.  Combat is a mess and has a lot more to do with who came in the door first and how fast people came in behind them.  

Fog of war is real, and should work both ways, so the initial rounds I always play as both sides groping around to get a sense of each other as situational awareness grows.

I agree 100%. In our Champions days, at first the villains didn't know that the girl in the black spandex with the red star on her chest and the red face mask had mental powers... but after the hero team became famous and made the news enough times, eventually all the villains knew that Psiana was a mentalist and hit her first.

But without something like a news report or recon, the goblins or orcs won't know what that halfling can do until the first couple of rounds go by.

I can't remember which comic writer but it might have been JM Dematteis in his parody-laced JLI run back in the late 80s... someone had a non-hero character ask one of the superheroes, why the heck they name themselves after what they can do, instead of being more coy. That is... If you have fire powers, why call yourself Flame Lad? Why not call yourself Ice Boy and then the villains will expect ice and prep for ice and you can get the drop on them. 

Similarly, the halfling barbarian could dress/act like a thief, and then suddenly WHAMMO... Rage/slash/kill. And if the players do that, I would say the DM should at least make it possible via failed Insight checks or Int checks or something, that the NPCs would not realize they are being played. At least at first...

I'd put some feel-good encounters in where the enemies make some really bad decisions. Presumably this player wanted to RP a halfling barbarian because that's unexpected, and yet they are getting the same treatment as any barbarian. Instead have one of the bad guys say something like "I'll take the little guy", and then let the halfling beat him down. Have another confront the wizard and be like "look at this guy, he hasn't even got a sword!".

Tactically, if you think something is a small threat and easily dealt with, you send one or two guys to deal with it, not your entire force. Then they might realise 1 or 2 guys was definitely not enough for this small ball of anger and blades, and that's going to have bad effects on the rest of their plan. In the same way as if the fighter says "I'll kill the ranger", takes a swing and the ranger dodges his sword and then turns into a were-bear. "This complicates things".

Quote from MidnightPlat >>

I think the tactic being disputed presumes a sort of almost "fight card" knowledge level, where the opponents have weighed in and analysts have watched their last performance metrics.  How much reconnaissance has the enemy actually performed on the party who's just walked in on them?  Fog of war is real, and should work both ways, so the initial rounds I always play as both sides groping around to get a sense of each other as situational awareness grows.

Prepared attacks by opponents who've done their homework that gives them the tactical insight some DMs are presuming for their NPCs/Monsters shouldn't be that common.  Combat is a mess and has a lot more to do with who came in the door first and how fast people came in behind them.  

 Absolutely true.  In this case, the mystery is SUPPOSED to be that the goblins are getting high-level training from somebody, and party needs to find out why.  Perhaps too subtle.

Quote from ThorukDuckSlayer >>

I'd put some feel-good encounters in where the enemies make some really bad decisions. Presumably this player wanted to RP a halfling barbarian because that's unexpected, and yet they are getting the same treatment as any barbarian. Instead have one of the bad guys say something like "I'll take the little guy", and then let the halfling beat him down. Have another confront the wizard and be like "look at this guy, he hasn't even got a sword!".

Tactically, if you think something is a small threat and easily dealt with, you send one or two guys to deal with it, not your entire force. Then they might realise 1 or 2 guys was definitely not enough for this small ball of anger and blades, and that's going to have bad effects on the rest of their plan. In the same way as if the fighter says "I'll kill the ranger", takes a swing and the ranger dodges his sword and then turns into a were-bear. "This complicates things".

 That's a great idea!  Should thought that in the first place.

Absolutely true.  In this case, the mystery is SUPPOSED to be that the goblins are getting high-level training from somebody, and party needs to find out why.  Perhaps too subtle.

Not necessarily too subtle but it sounds like it is taking too long to develop.

Quote from BioWizard >>

Absolutely true.  In this case, the mystery is SUPPOSED to be that the goblins are getting high-level training from somebody, and party needs to find out why.  Perhaps too subtle.

Not necessarily too subtle but it sounds like it is taking too long to develop.

+1^x.  I get wanting to keep the mystery, but you need to literally smack them with some of the clues.  When you fluff your combat narration riff on the "only Imperial Stormtroopers are this accurate" trope.  Tell the martial characters, "in all your training and/or combat experience you've never seen goblinoids attacking with this sort of precision efforts, some of the weapons handling you're witnessing seem like techniques out of some of the more elite military academies of your world."  The rogues get "while skulking about for opportunities to exploit, you can't help but notice that these goblinoids ... are just good ... there's an intentionality and attention to strategy and tactics you're not accustomed to when working over these types."  NPCs conveying their impressions ain't cutting it, so making this a matter of the PCs directly witnessing it while being cut up will hopefully do the trick and ignite the party's investigative drive.

It's OK to draw some things out, but when you have bog-standard monsters acting in unusual ways, you need to find a way to let the players know that this is part of your storyline and not just some arbitrary thing you are doing. Even if you have to tell them outright.

For example, when I started my campaign a year ago, I decided to do something cool and special with Psionics since D&D had really not done anything. I therefore told them, "No Psionics of any sort allowed." When Tasha's came out, I disallowed anything Psionic in it. But I also said to them, "I'm doing this because I have a cool custom idea that I think is better and fits my world more than what Tasha's did." I didn't just not say anything and wait till they encountered my world's Psionics (which they are now about to do, I think... 25 sessions in).

Okay let me get this straight the halfling barbarian who I assume has decent Dex and Con and should have around 14-16 AC if  they don't employ a shield and go with the two handed weapon.

First hit wouldn't they be able to use their reaction to employ rage as that's a bonus action thus halve all damage that isn't magic based?

Any goblin making the mistake of dog piling the barbarian would be in for a bad time unless the DM got some lucky rolls.

More importantly why would they target him since wouldn't they focus on the weakest member and that isn't going to be the barbarian in any sense of the word!

You can't use a bonus action as a reaction.  Bonus actions can only be taken on your turn.

Quote from 6thLyranGuard >>

You can't use a bonus action as a reaction.  Bonus actions can only be taken on your turn.

What if I ready rage?  I kid, I just like the sound of readying rage "I take a deep breath and am prepared to be set off by the slightest provocation."

"I log on to Twitter."

Quote from TimCurtin >>

"I log on to Twitter."

You mean your device of mutual vicious mockery (or dissonant whispers if you fail your save again doomscrolling).

So its literally possible for a group of 6 goblins to bundle the halfling and neither the halfling nor none of his comrades can do a thing about it?

Damn that's cold!

If an enemy beats you on initiative, then yes, generally they get the opportunity to pummel you before you can react.  That's what initiative is all about.