D&D 5E 5e Assassin build Advice

[FrogReaver]

Pass without trace is the single best enabler for surprise rounds. This spell will enable your auto crit feature with the rogue. Ranger is the best path for you to get to Pass Without Trace as it grants extra attack and hunters mark. After that you take rogue till Assassain. I'd play with it in the campaign up to that point and then decide if it made more sense to invest more into the surprise round (fighter levels) or more into rogue levels for general damage and better skills.

 

[FrogReaver]

Any chance anyone could help me out with some DPR calculations? And ideas on how much is actually good/needed at any level?

The three early game builds I am looking at based on the advice given thus far.

Fighter 5 / Rogue 1 :
Pros = Seems to combine the consistency of the fighter class with the early utility of the rogue class; in the next two levels assassinate comes
Con's = No ranger levels until level 8 so levels 9-14 will see much less growth.

Fighter 1 / Ranger 5:
Pros = Get's the extra attack, pass without trace, gloom stalker (god tier imo) damage in turn one. Fighter 2 and Rogue 1 in the next two levels (followed by rogue 2-3) should ramp up quickly-ish. However;
Cons = No Action surge or Rogue though, so full assassinate nova will not occur until level 9 or 10.

Fighter 3/Rogue 3:
Pro's = Assassinate, expertise, action surge and battle master all there!
Cons = No extra attack until level 8, no ranger levels so 9 - 14 levels will be pain. All ASI come very late in comparison to single class

1	Fighter 1:	Archery
2	Rogue 1:	Sneak Attack, Expertise	Expertise Utility
3	Fighter 2:	Action Surge	<- enough said?
4	Fighter 3:	Battle Master	Extra damage when you need it
5	Fighter 4:	Asi
6	Fighter 5:	Extra Attack	Extra dpr

1	Fighter 1:
2	Ranger 1:
3	Ranger 2:	Hunters Mark, Fighting style (Two Weapon Fighting)
4	Ranger 3:	Gloom stalker	Extra attack on first turn and an extra 1d8 per hit.
5	Ranger 4:	Asi
6	Ranger 5:	Pass without trace, Extra Attack

1	Fighter 1:
2	Rogue 1:	Sneak Attack, Expertise	Expertise is a tick
3	Fighter 2:	Action Surge	Yep!
4	Rogue 2:	Cunning Action
5	Rogue 3:	Assassinate	Nova!
6	Fighter 3:	Battle Master

I realize this entire build is based on surprise damage and that surprise is not consistent. I'm kind of nervous about it, I don't know what damage I should be doing at end game per turn, I don't know what is normal/average.

For archery it will be hard to beat the gloomstalker ranger. Round 1 he hits really hard but not quite as hard as a battlemaster fighter spending all resources. He typically does quite a bit more damage than the fighter the remaining rounds due to hunters mark.