Combat help

Hey I need help with the combat system. So let's say a machine is allergic to acid , I use acid arrows and I have the white timer acid icon but then what ? Where do i shoot and what ammo ? I understand there are parts but when i shoot the parts somtimes they make big damage sometimes its not..feel like I'm just wasting arrows and materials for nothing .

Thanks for the help !

Yes its called weakness and not allergics but for me they aren't weak , they kicking my ass