To address the issues that Dekkah raised:

You can't shift while climbing, unless you have a climb speed. Mertes doesn't.

Creatures in the treetops always have concealment.

If you climb into the treetops on the same height as an enemy you negate the cover but not the concealment.

So case #2 applies; Mertes will eat an OA from the slinger (with CA since he's climbing, but that is more than cancelled out by the concealment and cover).

(1d20+6)[17](23) vs ac 16 for (1d4+3)[4]