per BlakeJohann on TrueTrophies
I found the easiest way to get this trophy was with the mosquito guys that launch green goo at you. they fill up the void shield quickly and you can easily get far enough away from them that the concussion from the shield release doesn't hit them but the returned missiles do.
There are three of these guys at the beginning of Torren IV. you can just take them out and then hop in the ship. leave the world, and then come right back and they will respawn.
You will need to have the Reactor upgraded to level 5 first though. I found the fastest way to upgrade it was to enable infinite ammo and then go to the last level of the gold arena, where you face the centipede and dinosaur bosses together. just use the reactor on the bosses over and over again and you can fully upgrade in 1 or 2 runs.
per Meteorain988 on TrueTrophies
Note: Make sure that before you fire back, you're far away from the enemy otherwise you just hit them with the blast rather than the projectiles