This achievement is obtained by assisting others in killing 25 hostile mobs. To assist, you must "maim" the mob by damaging it without killing it, and then someone else must kill it.
Master Sidekick Achievement
Assist in killing 100 hostile mobs.
The game defines "assisting" as when you "maim" the mob by damaging it without killing it. You can achieve this by shooting a small burst of your weapon, or even by smacking the mob with your pick. For this to count towards your sidekick achievement, another person must then follow through to kill the damaged mob.
You can also visit a populated mob farming world and simply shoot a few times into an enclosure that someone else is working at. Be sure to explain yourself first, though, as many players will interpret this as you trying to "steal" their kills, and may respond in an unfriendly way.