There is a world of difference between a non-zeta, g11 Phoenix that most would use in p5 TB or early on in arena, and a full zeta g12 Phoenix. Zeb and Sabine are completely transformed. I've had zeb one-shot more toons than I can count, and Sabine increases the whole squads damage output drastically from exposes and CC/CD up buffs while also stripping tons of TM from stagger. It's not unusual to start a wave and have all 5 toons go before the enemy even gets a single turn, just from kanan's foresights expiring at the previous wave's end.
For tw defense, they also force the attacking enemy to go in with a very high damage, fast squad; otherwise, Phoenix just keep healing over and over. You can't lowball them and expect a victory.