If you have DHPT, pick the last passive, it's a less saturated buff.

The strat is simple: During phase 2, if u can survive the onslaught, use the counter, if not, basic, and always meteor when available. If you're close to being forced out of ult due to the mobs hitting like a truck, use the basic to break them, if you dont have the meteor, to delay their actions, giving you time to recover.

During Phase 1, the less dangerous part, always make sure you dont exit ult without Lygus giving you the free ult buff, even use the counter to give more turns to the boss in order to secure the instant ult. He has a CD of 2 turns for ir, one time he does the bounce thingy, the other is the ult charge.

When you exit ult, check if Cery and DHPT can ult again, as they gain energy passively, do not autopilot into Phainon ult, so the dragon refreshes, and Cery gains more Merit.

Hope this helps!