I found 3 bugs with Colossus Slayer using a Longbow that I've reported:
1. It applies damage on the first hit (when target is full), which I think isn't suppose to happen.
2. It doesn't stack with Hunter's Mark (at least in the fight where I noticed it, Hunter's Mark would add damage and Colossus Slayer would be omitted from the combat log). It's possible this doesn't always happen, I'd have to go do some tests, but I checked it over the length of a medium fight and it did this every time.
3. It gets applied to Knock Out which is sort of hilarious, killing the target.
