You will first encounter this during the level 'Fireball Fury' in Act 3. They are harmless and just slowly walk across the map so just stand underneath one and unleash as much firepower as you can. They have a huge amount of health so just keep blasting away at it's underside until it dies. You will still be attacked by random enemies so beware of them at the same time. There are more than one of these, so you can kill whichever you want for the achievement. However the second one you encounter is good protection from some Fireballs that are landing on the map, so you may want to let this one live and use it for cover. The best one to kill is the first that you encounter.
As soon as it dies, the achievement unlocks.