Gungrave G.O.R.E

Storm Barrage is the one of the only skills in this game that has a prerequisite.

Once you get a beat score of 50+, you can launch it by pressing .

I'd recommend finding a group of enemies that has riot shield guys. What you'll want to do is kill all of them EXCEPT a single riot shield guy while ensuring your beat score is over 50. Then what you will do is get up close to the guy and tap once, wait for the animation to end and then repeat so you can chain activations while keeping a combo going. Once your shield goes down you'll then want to back up and start shooting the riot shield until the shield recharges fully and then repeat the process. Since you are shooting an enemy nonstop throughout this cycle your beat count will never leave so you can easily farm the 100 and 500 activation achievements in this one encounter!