To get this achievement, the Rhino must take damage from ONLY pistols. This means that the Rhino CANNOT take any damage from turrets, sentries, vultures, traps, grenades, weapons, SOFLAM, etc… Also, only the player who delivers the final killing hit to the Rhino will get the achievement. It is recommended that you do this with a party of 4 players who you know, or solo. If any player damages the Rhino with something other than the Pistol, the achievement is voided for everyone.
The easiest place to get this achievement is directly after clearing all the hives in Area 2. After clearing the hives, players want to make sure that they have no offensive abilities active, and that traps have been disabled. Now, once you enter the hallway that leads into Area 3, a single Rhino with no other enemies will spawn. Lure the Rhino into the open area and have all 4 players unload on it using their pistols. It is recommended that you use the automatic pistol with explosive ammo, and it should go down within 40 seconds. Once the Rhino dies, only the person who hit it last will get the achievement. It should be noted that this achievement can be done solo also.
The achievement is also possible at other times during Area 2. The first Rhino will spawn from the ground during the 3rd hive of Area 2. It is advised that you immediately turn your sentries around, or don’t deploy them in the first place for this achievement. Also, using a flare will help take the attention of all the other smaller aliens that get in the way and damage you. Have the team focus fire to kill it as fast as possible before other enemies get in the way too much. A video guide of this method can be seen HERE.