That, and when you do, Celestasia usually isn’t your main concern. I run Celestasia in my Dragon line-up, but if someone can manage to kill the other three, I highly doubt she would be a problem to finish off. If done right, that line-up kills you, not the other way around, and quickly at that. If someone is good enough to get past Venoxia, Emperina and Bone Drag, then they really should be skilled enough to take on a creature that has no damage to speak of.
See also that other guy QQing that it took him an entire hour to beat Celestasia on her lonesome, with his full-blown Goblin team LMFAO. But hey, it’s not about his skills! It’s about BALANCE! The creature is OP! 
The funny thing is that, before the recent balance change, Celestasia used to be able to drag games on for a long, long time, and we seldom saw complaints about it. No, it’s now that she generates less gems, and that the devs implemented a code to limit 4-turn loops that people start crying wolf against her looping ability…
Go figure. 
Also, the AI absolutely sucks at playing her. It typically wastes her spell on her rather than buffing her stronger allies.
Again, Celestasia used to be barely decent, and very situational. “Restore Life” has always been a weak ability, unless you combined it with “Gain Life”. Now, she’s fairly good. But overpowered?! Not in a million years.