Acro-ing refers to the use of the Acrobat Bomb to alter gun performance. It is known to be used with the Afterburner Gun and the Gatling Gun. One can deploy the bomb/pod to send yourself sky-high, then quickly fire the gun so that the shots slide vertically as you descend. This is possible only on Wi-Fi.
Another popular method involves using an air-jumping robo, Acrobat Bomb, Umbrella Pod and Glider Gun. You can short-hop-acro-glider into the air for cover fire, then release an Umbrella Pod. If you also fire the gun, it's an almost inescapable tactic.