What is the loot at 3 stacks and 4 stacks?
If it remains the same, than what is the point in engaging him during that time?
Not exactly sure on the loot table, but it seemed like each stack higher saw an increase in loot. More black hole pieces, 2 dread mask instead of one, etc.
And the only way to fight him at more than one stack is to farm the dreadful essences and apply them before the fight starts to buff him. The whole he gets stronger each time he dies isn't working, at least in our experience we'd kill him and he'd come back 30 minutes later still at 1 stack....