Colossus Slayer only triggers if you attack a target which doesn't have it's full hit points (meaning they took damage in this fight already).
Would damaging the target with an Attack Action make it possible to then use Colossus Slayer with a bonus action off-hand attack because the opponent doesn't have full hit points anymore?
This would basically mean that a ranger could use Colossus Slayer on nearly every turn except if the Attack Action misses or kills the opponent.