This one is really easy! Once at Rank 17, you will unlock the ability “Counterattack”. This gives you unlimited Grenades, Molotovs and Flash Bangs. Just buy three or more if you’d like to. Then run head first into a Zombie, the Counterattack move only triggers on Zombies and when you’re grabbed while facing them, then just press X when prompted.
To confirm, Dog, Bioweapons don’t trigger the Counterattack, but Zombies and Ivys do.