I think a lot of us have screamed something like this at the screen

"This isn't even my final form!" he screeched, a maniacal laugh echoing through the room as his body began to twist and change, becoming larger and more monstrous with every passing second.

His new, less aesthetically pleasing form was not improved much when I hit him with my most powerful attack.  In the face.  Twenty times.  (Or maybe it was improved.  Depends on your point of view.)  "It's your final form now" I calmly told his corpse.

Then I incinerated him, split the ashes into twelve redundantly sealed containers and shot them into the sun.  And we all lived happily ever after.  Except for him, because he was very dead.  The end.