Would suggest that you send your gimp tank to take the boss outside (Kardis) she doesn't hit hard unless you fuck up the slimes but it largely depends what class you have available.
Also, ignore everyone saying "Herpa derpa midwinter etc" they didn't use "the 2 tank tactic" they used the "stack them up and cleave" tactic. Two tanking is much harder than three tanking for sure, but doing it sensibly (separating the bosses) does not make it damn near impossible as people are suggesting, having the bosses on top of each other as midwinter did is just insane and not really anything to base your judgement on.