Testers are the pink enemies found on the final level. The best place for this trophy is in the room directly before the final boss, where you fight a group of enemies, half testers, in a small area.
There's a trick to this that make's this trophy WAY easier than usual, that I found while on my THIRD run of the game (twice on the 360 version, once on PS3).
If you use the flame power with the grenade launcher, the grenades will explode on contact with anything, AND set enemies affected by it on fire. As far as I was able to tell (trophy popped too fast for me to really test), Testers killed by the fire still count for this trophy, even if you weaken them with other weapons. So, in that final room, kill all other enemies and wait for three testers to be close together. Fire the flame powered grenade where it will hit all three, and if the first shot doesn't work, fire one or more shots. Like I said, the trophy popped before I could test it, so you may have to do some testing of your own to find something that works. But with the flame power up, I don't think it'll take too long. :3