I act as the MT on Leo in Elf form and then the lock gets him for the Demon form.
I wear max threat gear because of the aggro dump between phases and after whirlwind. Therefore Im wearing all my expertise and SBV gear.
I allways stack up my rage 10-15 seconds before he shifts to demon. If I get a inner demon I switch to bezerker stance, pop recklessness and use my SS macro thats tied to the Auto Blocker. You will be able to get some nice 2k + Shield Slams in with 620 ish SBV. That will basically nail the demon well within the time you have. If I get another inner demon(wich hardley ever happens) then I repeat the above minus the recklessness(dam 30 min CD, lol).
There is something so appealing about backhanding someone across the face with a shield.