I found an easy way to kill them and get that Handy Acheivement....Grab the Super Reloader mutator and turn it on. Then shoot all your ammo out of your lancer until there's one bullet left, switch to the Scorcher (Flamethrower) Shoot him once until he grabs
his head like he's in pain, switch to your lancer and Active reload and Light him up. easy as 1,2,3. I did wave 1-50 on Insane by myself with the super reloader mutator. some might say it's cheap but whatever. least I did it.