This is one of the worst designed enemies in the entire game: it's either a nothing burger or impossible to deal with depeding on how late it spawns in the scenario. It also punishes smaller groups harder than bigger ones because it's far more likely for the same Investigator to be carrying a bunch of guest on themeselves.

Normally, enemies that get buffed stats have an upper limit to how strong they can get defined by the game. The Shantak doesn't: it can get a total of 14 Fight and Evade if you have gotten all guests in the same location, which is such an insanely huge value it's nearly impossible to overcome during a skill test in normal circumstances, and it's only an uphill battle if you are playing on a higher difficulty. At that point, you can only use testless evasion and damage to take it down, and even then 4 health makes it bulky enough that it will persist for quite a bit too.

Just really shows how quickly Midwinter Gala got shat out.