in Normal mode. all bosses are easy mode. there isnt any phases. quick 5min fight
Nightmare mode. after boss hits 50%hp should take 5mins.then Phase 1 starts boss changes does new spells. maybe summons adds. he does this untel died... 5mins for phase 1 also. 10min fight
Hell mode. when boss is at 75% Nightmare mode phase 1 starts. then hes at 50% Phase 2 starts.. he does crazy Hellish spells. alot of AOE to run out of or you die. 15min fight
Inferno Mode. Phase 1 starts at 75%.. nightmare phase.. Phase 2 starts at 50% Hell Phase
Phase 3. at 35%. Crazy Inferno Phase.. this is were the fight gets really hectic. you gotta be smart or you will die in 2-3 hits
think about if the Skeleton King worked like this... maybe Inferno mode. Phase 3 he turns into red flames. and summons fire skeletons that shot fire arrows. and leaves fire on the ground that we cant stand in.