exact0's all-purpose description above is great - set up your game with the parameters he lists at the top of his solution to get the maximum opportunities for multiple achievements out of Prey multiplayer.
Yet, here are a few ideas for more fun and efficient ways to get this particular achievement for the 25 Crawler Launcher and Crawler grenade kills.
Set up the game as exact0 suggests, with the Player Team Deathmatch. Use the map Roadhouse.
The launcher is in the upstairs bathroom. To get there, go up the broken stairs and take the door, follow the hall (there is a crawler grenade in the left hand corner) through the next door, there is the bathroom door on your right.
There is also a portal that leads to the outside if you keep going straight and miss the bathroom door on the right.
That portal leads directly into a crawler grenade stash as well.
You can agree you and your partner will ONLY USE the crawlers and the crawler launcher, though that isn't really necessary since these guns are compatible with others in the game for a fun experience, unlike the wrench.
If you are going for the crawler grenade achievment, be sure to hit the left trigger to make the crawler a mine, not a grenade, that way you can plant them places and make your enemy walk into them when coming through these many doors, the portal, etc.
It will go quickly enough and be a lot of fun.