For what it's worth, I like Flash for Turn 4 for the reason Giantbane mentioned: It uses a different resource.

For the fourth phase (2x Spinner Rooks, 4x Clockwork Bugs), conserving TP by using some mixture of Overpower and Flash is better AoE damage and better enmity than using only Overpower and conserving TP by filling with single-target attacks and standing still. I'm able to enter the fifth phase with almost full TP, and go absolutely nuts in the sixth phase with all my offensive cooldowns and Overpower.