Gungrave G.O.R.E

First of all you should do this on your first playthrough on easy when enemies do negligible damage. Just upgrade your auto target range but NOT you bullet damage (obviously also your shield regen). Starting about Stage 15 you will see more and more enemies get a RB prompt above their head when shot a couple times. Do NOT RB them but instead walk up to them (you have ample time as the stun lasts very long) and then press the left stick...this counts as an execution. Do this 50 times and then 100 for two achievements. After that you can start pumping bullet damage to your heart's content.