FO Full Zeta Squad -- Who/When to replace?

The biggest advantage of using fox over fotp for arena (especially on defense) is simply durability. Fotp is super squishy, and any aoe or expose heavy teams ( pretty much any current meta) will quickly wipe him out. Fox will quickly ramp up his Max health, especially if you mod him correctly with a lot of health primaries/secondaries. His offense is directly tied to his Max health, so the usual attacker setup of offense and CC/Cd isn't the right way to set him up. A CD and health set, and health primaries on the cross, triangle, and circle will have just as much damage as using an offense cross, with more durability. With FOST's zeta, you will be getting a lot of assist calls that will ramp him up even quicker.