When it's in plant form, you've gotta shoot the tentacle bit that's shooting at you. When it's human form you've gotta get headshots while it's charging its attack
It was fine for me when using assault rifle. Probably took 3 magazines straight into the head. There's a different audio that plays when you get headshots vs body shots. Body shots will still give bloody but headshots sound different which you need to kill it.
I only use ammo on bosses, you should never use it on anything else really. Your stick is good enough for all enemies until you reach the grey runners.
