You don't really need to shoot them or use the knife or anything. Just stomp, and if they're stunned, target them, take a step back, and use unarmed R1. Kills them 100% of the time once you know the distance. You have to distance yourself so that you step a bit further than they are with the R1's step.
Lifeshrooms are really good in that not only can they save you from death, they are one of the few items that heal a percentage of your life instead of a fixed amount, so they're great once you health pools start really climbing.
Other good items for healing with a high health amount are cooked yogashroom (heal for a little under 1000 over 40 seconds and refills the rage meter, great for healing after an encounter gone wrong and I always carry one) and gambleshrooms (50% chance to both heal and fill the rage meter to full, 50% chance to turn into a vulnerable frog, again best used outside combat situations for obvious reasons).