Well, assuming you see Mewtwo as faster, Seph still has a 2HKO which he can keep up constantly. Supernova, Shadow Flare, physicals, Break, it doesn't matter. It's unlikely Recover is even enough healing, and even if it is, he's at best stalling until Mewtwo.
That's giving Seph free will with his AI. If you hold him to his in-game algorithm, that's okay, as long as you do that with every other boss too. Unless you're giving bosses who have a random chance to use any move a free pass compared to those that are more patterned, which seems blatantly unfair to the latter.