That's it. Team Flash need to kill Marlize. Devoe will be overcome with emotion and grief (if he still has any feelings) that he won't be able to think straight. He will make lapses in judgment and overlook certain things. Instead of thinking with his head and planning out his attacks, he will act out of pure instinct and rage.
Of course, I sometimes think that Devoe is about to kill Marlize himself now that she has outgrown her use. And especially after this latest stunt with leaving him and saying that he's nothing without her.
And why can't they ever use Cecile to combat Devoe? I know Joe wouldn't want to involve her, but she would be able to read his mind and know what he's thinking. She could just be within proximity during a battle and then relay over earpieces what Devoe has planned and how to avoid it.