So when you go through phases of killing Diablo. During the cut scenes you can still attack just keep.spamming your attack while holding down the shift key
if done right in the 2nd phase Diablo will come downi go up you will see the shadow people then it sill exit to the 3rd phase
You need hefty dmg to do this I'm doing 75k this can be done in normal nm and hell
Sometimes its tricky you need to spam your attack also the more the merryer.