Help, My Players Have Taken It Too Far: I Need Help Making A Sans Goblin Boss

[Valorium]

See title.

My players have inadvertently made a meme out of a particular goblin that rolled very well on avoiding attacks (alongside the fact that he coincidentally had blue leather armor on) nicknaming him the "Sans goblin". Now (because I have poor self-control) I have established the expectation that there will be a future floor boss (I'm running modified Dungeon of the Mad Mage, because I have a death wish) that is Sans, from Undertale, but a goblin.

Please help me design this aberration against nature.

I had a few ideas – they start the battle by casting Telekinesis, maybe chuck some people around or throw scenery around. Give them high AC and maybe some kind of Evasion or Uncanny Dodge effect, but low health. Legendary Resistance is a must. Any other ideas? I've not established any sort of firm time wherein this fight may occur, so there's no need to rush.

If you're familiar with DOTMM, then I'd also appreciate input as to which floor this boss should be on (and therefore what CR it should be), with the condition that the floor shouldn't have some kind of imposing floor boss as it is.

[LudicSavant]

See title.

My players have inadvertently made a meme out of a particular goblin that rolled very well on avoiding attacks (alongside the fact that he coincidentally had blue leather armor on) nicknaming him the "Sans goblin". Now (because I have poor self-control) I have established the expectation that there will be a future floor boss (I'm running modified Dungeon of the Mad Mage, because I have a death wish) that is Sans, from Undertale, but a goblin.

Please help me design this aberration against nature.

I had a few ideas – they start the battle by casting Telekinesis, maybe chuck some people around or throw scenery around. Give them high AC and maybe some kind of Evasion or Uncanny Dodge effect, but low health. Legendary Resistance is a must. Any other ideas? I've not established any sort of firm time wherein this fight may occur, so there's no need to rush.

If you're familiar with DOTMM, then I'd also appreciate input as to which floor this boss should be on (and therefore what CR it should be), with the condition that the floor shouldn't have some kind of imposing floor boss as it is.

High saves. Reroll mechanics (like Lucky). Contingency. Shield and Silvery Barbs. High AC and inflicting Disadvantage to be hit. Nondetection + bonus action stealth + vision control. Counterspell. Evasion like you said.

[Ganryu]

Be careful to make it fun and winnable. I had a boss once that was an illusion, but once they found the rune the illusion was cast from, and destroyed it, they were able to win, and it was creative.

Make ways they can win without hitting the boss. Add some stakes. Have something happen when they DO get a hit off.

Rule #1 is make in fun.

Final boss of one of my campaigns was just a health sponge, and nobody liked the fight. He didn't do enough damage to be interesting, and whaling on him wasn't enjoyable. He had a fun AC of 10+1d20 every round, but once that gimmick was discovered, it was the last interesting part.