You do know you can prebuff characters right? And enemies don't.
I finished it on tactician, there was never a scenario where phoenix dive overpower couldn't knockdown, not EVER (unless the enemy was immune but that is an entirely different story.), overpower doesn't need to crit, it only needs the character to have more armor than the enemy (so always). I was running only one character with it though, the game is already easy enough.
And don't get me started on how enemies can only try to keep up (and they still fail) by cheating both the turn order and "loremaster".