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).
I actually found it working most of the time and with HM and not working sometimes. There is certainly quite a few bugs with CS at the moment.
And yeah... lul... Rangers are quite a tad far from being OP, though I believe they might be a bit better than usual in BG3, because so many ledges and high ground positions all over the place make it very easy to get Advantage on ranged attack. Though I bet Paladins will be still by far most ridiculous especially with weapon skills giving them free AoE attack, even if only once a short rest.
