This should be pretty easy/cheesy to get in skirmish mode. The way I got is, I modified the Victor mech, by stripping out it's armor/weapons, and just giving it a machine gun. Then I started a skirmish match playing with 4 Panthers, against the crappy, modified Victor. I ran up to it with my 4 Panthers, meleeing it until it's CT was destroyed, and unlocked the cheevo.
Even the armorless/weaponless Victor hits pretty hard, so you could lose a Panther or two, and I think you need to score the killing blow by destroying it's Center Torso or Head, just injuring the pilot until they're incapacitated probably doesn't count.