Yes exactly the problem I had. I unloaded a thousand rounds into the cores on his back and front, just thought he had a million hp or something. But unlike the giant crab that released ammo-mosquitos, there was no way to get more ammo. The problem is that few would think to get close to him to trigger the button sequence.

Guess I could have figured it out if I wasn't so frustrated while playing. Since he spasms after you hit his chest target, and further shots do nothing while he's spasming, that should have told me there was something else I had to do. Oh well, I enjoyed killing him.

Playing stage 6 on veteran using Sheva. It's weird shooting from the left shoulder angle.