Ok, here's what we got.
Fight above the overgrown crypt or whatever it is called in English.
Started with 8 points into level 2.
Killed two out of five enemies with environment mechanics: still at 8 points.
Killed the two enemies the "normal" way (shooty, shooty, hacky, hacky, clicky, clicky) in a different game yesterday or so: got XP for both kills.
Also from another game I played today I noticed that killing the enemies with the environment mechanics did NOT lead to their bodies being where they are supposed to be afterwards.