How to Complete 'Tis the Season
The objective is to buy time for four rounds. It is forbidden to have HP less than or equal to 20.
You just need to survive for four rounds. The only way to fail this quest is if all of your units die before four rounds have passed.
If you kill any Mimics, new ones will spawn to replace the ones you kill. It can be a good way to get a lot of loot, but on the other hand (no pun intended), if you keep making new Mimics spawn, it will take a bit of effort to keep your units' HP from going down too low. You might find it easier to just inflict debuffs on the Mimics such as Stop, Stone, Disable, Immobilize, and so on. That way, you can just wait for four rounds to pass and not have to deal with so many Mimic attacks.