Try human varient 4 level rouge scout. Feat crossbow expert then sharpshooter. Scout gets some nice benefits. Getting into and out of combat. Hand crossbow. Gets the bonus attack. Then 3 levels ranger gloom stalker. Give dread ambusher.  Then 2 level dip in fight for action surge.

You get surprise. 1st attack- 1d6 weapon 2d6 surprise(rogue) 1d6 hunters mark. Plus bonus sharpshooter= 4d6+14 crit=8d6+14, now dread ambusher 1d6 weapon 1d6 hunters mark 1d8 +14, then  action surge 1d6 weapon 1d6 hunters mark +14 dread ambusher is reactivated 1d8+14.so 5 attacks

1)4d6+14(crit) = 8d6+14=22-62

2)2d6+1d8+14=17-34

1) 2d6+14=16-26

2) 2d6+1d8+14=17-34

Bonus action 2d6+14=16-26

83-182 at 9th lv total with little magic. And 1 attack plus bonus attack using dread ambusher. More than likely if attack this you have surprise and get all attacks at advantage

Sorry, late to this thread, but had questions on Dread Ambusher and found this.

Question: rakassan101, how do you get Hunter's Mark on a target and still get surprise? I always wondered if HM alerted the target that you cast a spell on them since it says you "mystically mark" a target. Or if you have it up already (after initial target died and you are still concentrating on it) can you shift it to a new target without them noticing it.

Or, if just a regular encounter, on first turn of combat, you could cast HM as bonus action, move to target with extra 10' of movement, take attack action, do weapon damage + mod + 1d6(HM), then get the additional attack for weapon damage + mod + 1d6 (HM) + 1d8? Do I have that correct? 

They don't know they have been marked. As you level up it lasts longer so you can track your mark.

Quote from rakassan101 >>

They don't know they have been marked. As you level up it lasts longer so you can track your mark.

 RAW Hunters Mark has a vocal component. If you've got a gracious DM it won't be an issue since:

PHB 204

Unless a spell has a perceptible effect, a creature might not know it was targeted by a spell at all. An effect like crackling lightning is obvious, but a more subtle effect, such as an attempt to read a creature’s thoughts, typically goes unnoticed, unless a spell says otherwise.

But they can justify a stealth check on your casting depending on the circumstances. 

Quote from rakassan101 >>

Try human varient 4 level rouge scout. Feat crossbow expert then sharpshooter. Scout gets some nice benefits. Getting into and out of combat. Hand crossbow. Gets the bonus attack. Then 3 levels ranger gloom stalker. Give dread ambusher.  Then 2 level dip in fight for action surge.

You get surprise. 1st attack- 1d6 weapon 2d6 surprise(rogue) 1d6 hunters mark. Plus bonus sharpshooter= 4d6+14 crit=8d6+14, now dread ambusher 1d6 weapon 1d6 hunters mark 1d8 +14, then  action surge 1d6 weapon 1d6 hunters mark +14 dread ambusher is reactivated 1d8+14.so 5 attacks

1)4d6+14(crit) = 8d6+14=22-62

2)2d6+1d8+14=17-34

1) 2d6+14=16-26

2) 2d6+1d8+14=17-34

Bonus action 2d6+14=16-26

83-182 at 9th lv total with little magic. And 1 attack plus bonus attack using dread ambusher. More than likely if attack this you have surprise and get all attacks at advantage

 An alternative build I thought of a while ago is 3 lvls Rogue Assassin, 5 lvls Gloom Stalker, 2 lvl dip fighter. Comes online one level later but just need the Sharpshooter feat, Archery fighting style, and a long bow to deal massive damage in that first round. As now you have extra attack so action surge gives you 6 attacks for that first round. With prof + Dex + Archery you should easily have a +4 to hit even with the -5 from Sharpshooter, add the advantage from Assassin and it's nearly impossible to miss. And you don't have to risk surprise by casting hunters mark before the fight. 

1st attack) 1d8 + 3d6 + 13 (assuming Dex is +3)

2nd) 1d8 + 1d6 + 13

3rd) 2d8 + 1d6 +13

4th) 1d8 + 1d6 +13

5th) 1d8 + 1d6 +13

6th) 2d8 + 1d6 +13

Grand total of 8d8 + 8d6 + 78 = 94-180  and if you get surprise you double those dice 16d8 + 16d6 + 78 = 110-302