You need to kill 20 enemies while climbing a wall, or hanging from a catwalk. Basically if Drake is hanging with one arm and shooting with the other, it should count.
You can farm this easily in chapter 19, or any time you're able to shoot from a hanging position and kill guys. If you want to farm this, kill all the enemies you can and then restart from checkpoint and do it again until it pops.
There is a related trophy
that you may (or may not) be able to get at the same time if you're using the restart trick.
Note: Grenade kills of course won't count for this, but can be farmed in the same place if possible.