Alright so I ran some numbers.

A bugbear At level 14, with 3 levels in gloomstalker and 11 levels in fighter, I think you have the highest possible first round nova.

Note the phrasing of the dread ambusher ability: whenever you take the attack action during your first turn of combat. This means if you use the attack action twice, it allows you to:
Attack 1
Attack (dread ambusher 1)
Attack 2
Attack 3
Action surge
Attack 4
Attack (dread ambusher 2)
Attack 5
Attack 6

Giving us 8 attacks, two of which are empowered. You should expect to surprise your enemies as an assassin- and given that you roll initiative at advantage if you're using the revised ranger, and get to add both your wis and dex to it, you are very likely to get another 3 attacks. All of them at advantage because your target hasn't acted yet. You should be able to, with some luck, kill anything below a CR20 within the first round, or at least wound it so severely that your team can easily finish it off.