And I have another observation. Colossus Slayer is trigger only by main attacks, more precisely the attack buttons on the left side. Bonus action attack with off hand doesn't trigger it. The most interesting part is this: When you dual wield, the main attack button does in fact use both Action + Bonus action and performs both attacks. This will trigger Colossus Slayer. But when you attack with Bonus action first and separately, the main attack button becames disabled. You are still able to just left click on enemy and it does perform standard attack action but since the standard attack button is disabled, it is not counted as true standard attack action and Colossus will not trigger.
