The trick here is to let his poise meter go down some time before the midway point of first phase. If you don't, you will VERY often get a stun with the last hit of P1, resetting his poise meter for P2. The best place to let his poise meter fall is during the big axe slam followed by an explosion, as it usually happens at a nice point in P1. Keep in mind that he has a 9 second timer before he starts regaining poise. It's actually usually fine if his poise goes all the way back down to 0, as long as it happens before halfway thru P1 and he doesn't bleed any more poise later.
Also, you have to get an R2 before the critical hit in P2 or he won't automatically phase transition (at least with these stats, but you're very unlikely to have much better damage than this with the Reduvia).
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