For this achievement, there are several suggested methods from the Xbox One TrueAchievement page.
First, for any of these methods, you need to make sure that any Skills, Shields, or whatever you have equipped will NOT deal any passive damage to enemies, since you don't want to kill them with anything other than your slam.
Second, the Oz Kit's slam element CAN be whatever you want, though if you have a Cryo slam that would be choice just in case.
One method is to go to Stanton's Liver (no fast travel, accessed from Triton Flats) and head to the far south-west area. A bunch of Toroks can spawn there and there are lots of Cryo Vines. The Toroks are more than happy to chase you down, so just lure them to the vines, let them freeze, then smash 'em!
Now, I'll note that this method is how I did it, but it took a few tries, as the Toroks kept attacking me and the knockback made it so I couldn't really jump to slam, then Cryo Vine would freeze and do enough damage quickly that it killed them off before I could succeed. I finally managed by jumping up on a small ledge by a couple Cryo Vines in the center of the room so I could wait and jump without interference.
This may work better on True Vault Hunter Mode, due to enemies' higher HP... though that's exactly why I didn't want to do that. Your choice.
Also, there are other locations that can be good, such as Outlands Canyon.
Another method is to get a lower-level Cryo-slam Oz Kit, then just go anywhere Toroks spawn (such as Stanton's Liver or Titan Transportation Hub), let them gather around you then just repeat jump, slam, jump, slam until they are all Frozen and then dead.
This may not work if you are too high of a level, causing the slam to deal immediate death rather than freezing first.
Alternatively, use this same method, but on True Vault Hunter Mode to keep enemies at good HP ranges, though maybe not as easy to quickly kill with the slam either, possibly risking them thawing and moving.
An interesting third method involves a Mission Item. During the mission "In Perfect Hibernation", you are given a weapon called "Lazlos Freezeasy", which has a very low damage output (a mere 8 damage!) but high freeze. So grab that, head out to anywhere, cluster three enemies together, freeze 'em, and jump 'n slam!