This one certainly requires a bit of luck. It requires you to gain 5x Combo, 5x Blast and 5x Air at the same time. This means you will have to kill 5 enemies with a gun (You could also melee-kill them) while standing on the ground, kill 5 enemies while in the air (Also no explosives here) and kill 5 enemies with any kind of explosives (Grenades, Rocket launcher, grenade launcher, etc.).
A simple technique is to throw a grenade into a big group of enemies, jump in the air, shoot 5 guys, land, and kill another 5 enemies.
A good place for doing this is Hope Springs, zone 4, wave 4.