What I've found that works is, once you get him into that downed position, you have to walk up to him and act like you normally would when you take him down, even if he gets back up. You gotta make sure (or at least hope and pray if you're in a lobby of randoms with no voice chat) that your teammates stop moving so that Antheus will stop as well. As long as Antheus doesn't move, you should have enough time to run up and start the execution animation.