Foxes can't even dent any of those bosses.. ):
Bring some friends with you or use melee.
Sad truth is that without guardian, 100k+ HP, fast legs, lot of AP, attribute, LK, potions and scrolls, Tartarus is too deadly.
Did anyone try resistance equips? Like electricity, electroshock and dark resist against Gaither's attack, shadow resist (because it looks like they are all shadow monsters)?