Interestingly enough, I went back and checked, and not only is the combination of hunter's mark and colossus slayer mostly working now, it seems this combo actually prevents colossus slayer from working on full health enemies. In my (somewhat limited) testing I did over the past ~30 minutes, the only time colossus slayer still triggered on a full health enemy when hunter's mark was up was on a crit. When hunter's mark is down, colossus slayer triggers even on full health enemies. This is different from the last time I checked shortly after EA release, so I suspect they are fiddling with it.
Maybe they silently fixed it? I already saw some fixes lately, e.g. better skill list in character creator. I guess we should include EA build number to all bug and feedback reports or it will be total mess lately.
