Usually I feel like I can beat him on day 4 just before the low demon incarnation. I guard his first hit, attack his windup, guard his rampage, guard once during his exhaustion to line up a critical, hit his second guard, then critical his weakened state because on turn 3 he attacks. If that hasn't killed him, one final hit during his windup usually seals the deal, and if he lives even that, a his rampage will fail and you get a free finish.

However after downloading the latest earlier, my critical on his exhausted state was only dealing 1 damage.