Use a molotov. And ignore the grenade launcher. A molotov beneath your feet will see half a horde of zombies die and its mutants injured. It has a large radius and it last for around 10 secs. Perfect if you add in the max heal speed, since it means you get to heal yourself and powerup a girl without much risk, tho in this case its for you to kill muties, just make sure to run and gun them and use it when then form a large group.
And an overall guide to very hard mode. Get molotov for groups, and has for best weapons Phantom, Pulse Rifle and MAS12 are the only ones youll ever need, if you dont have them yet. Chainsaw, Denma and the flamethrower are good for waves and sniper rifles for killing mutants.
Its what i use for very hard.