Knock yourself down with your own weapon.

There are two ways to get this trophy.

When one of the Ghostbusters shoots his Blast Stream, shoot your own at his so they cross each other. It only takes a few seconds then an explosion will occur and push you both backwards, then the trophy will pop. The earliest you can get this is when you and Ray are trying to get Slimer at the Firehouse mission (Replay Mission: Hotel Sedgewick / You're Hired).

You can also get this trophy after you have gotten the Boson Darts at Time Square mission (Replay Mission: Times Square / Spin Cycle). Simply shoot a Boson Dart right at front of your feet so that the splash damage hits you too. You can't get the trophy this way if you buy the 'Boson Damage Immunity' from the equipment upgrade.