There are two stages to the battle. In the first stage Melvin has two attacks, a melee attack and a belly flop. Just lock on to him and send in your minions. Watch for a wiggle that indicates he's about to belly flop. When you see the wiggle pull your minions back of the belly flop will kill them all. There will be a short cutscene when Melvin's health reaches 50% and the second stage of the battle will begin. This time Melvin will roll around like a ball. You can send your minions off to the side to keep them safe and just keep an eye on Melvin and stay out of his way. Once he stops rolling around send in your minions and run up an attack him. Try and stay behind Melvin and he will slowing try to turn and face you. If you can stay behind him you should be able to kill him quickly. Otherwise you will just have to repeat the process a couple times. After draining his remaining health there will be another short cutscene and you will just have to run up and hit him once to finish him off.