To add some more, Rex can't do much in Phase 4. Effect of TM reduction and hp-dependent bonus Dmg on AAT is negligible, if not 0. Only thing that works is AOE Tenacity up to protect the squad from killing blow of airstrike.
The thing is that the status effect lasts 3 turns while cooldown is 4 turns, and AOE Tenacity up should be properly timed to avoid this short window with no positive status effect in action. Properly timing the AOE buff is quite tiring, and one bad luck can ruin the team. You might get used to it, but replacing Rex with Princess Leia is far more easy and efficient.
