Plenty of old threads about this:
Example1,
Example2. Looks like a Poizon should be one of the team, since she safely deals damage at range that doesn't produce counters, and she has her own strong recastable tank (Decoy). I still think a Bone Necro would be very effective to party with the Zon. If you go with a Poizon, then don't make the other char a phys-dmg build, otherwise Sszak will always stop you. Poison bosses are especially tough since their damage is very hard to endure.
In general, you need two builds that satisfy three key strategies of the boss battle -
Control, survive, and kill.