Lvl 50 Cresselia is rough but doable. Took me 3 tries to get it right, and I was angry at myself because I should have finished it the first time but did not expect the level's curve ball. As I said earlier, Solosis is your fifth wheel (although with very minimal presence early on, but I'll get into that later). At the very start Cresselia will encase either the far left or the far right column in barriers and then continue to do this every 3-5 turns. Solosis is useful for breaking these, but don't overextend. You want to balance inflicting damage with breaking the barriers, because when Cresselia gets to low health, what seems like a breeze spikes in difficulty. Suddenly, the amount of Solosis presence on the board quadruples and the moment Cresselia uses its previous disruption it's going to start casting semi-random barriers every 2 turns. It will actively avoid resetting the board by leaving you with at least one option every time.
The key here is right before that home stretch transition, to try and get a strong chain at the height of one of the earlier disruptions to push Cresselia to near-death (ideally you should have at least 3 turns left before Cresselia's next disruption). If done correctly, you will have a barrier-free board for long enough to finish the job.
Edit: Level 51 onward (excluding the coin levels) seems to be very lenient on the disruption timers and the disruption effects. It's much easier than the past 15 levels to be sure, even with the hp increase.