I read this a while ago and started thinking of how to create the Ultimate Assassin. How about a Lightfoot Halfling taking 6 levels of the UA Underdark Scout Ranger, then 3 levels Assassin Rogue and finally 11 levels Battle Master Fighter?
STR 8
DEX 16 (Increasing to 20)
CON 14
INT 8
WIS 14
CHA 14
Add 2 feats (Sharpshooter & Crossbow Master) and +4 to CHA through ability score improvements. If rolling stats instead, could add the Alert Feat instead of a bump to DEX.
Racial bonuses include the ability to reroll 1's on a d20, move through spaces of larger creatures and hide behind them.
Through the Underdark Scout, you'd have Advantage on Initiative, Archery Fighting Style, +4 to damage on Favored and Greater Favored enemies, +10 bonus to speed and an Extra Attack on 1st round of attacks, Dark vision and the ability to hide from creatures with darkvision, plus a few spells & other Ranger stuff.
Through Assassin you get 2d6 Sneak Attack + Advantage on attack rolls & critical hits on surprised opponents on 1st round through Assassinate.
Finally, Battle Master gives you another Fighting Style (+1 to AC via Defense), Second Wind, Action Surge, 7 Manuvers (Precision & Menacing Attacks are a must) with 5d10 superiority dice,
So, if you roll a higher initiative (with advantage) and surprise an opponent, you should be able to attack them with a crossbow 5 times (3 attacks via Battle Master, 1 via Underdark Scout -1st round only- and 1 via Action Surge). Using Sharpshooter (and Precision Attack if needed) you should land all 5 attacks. Assuming you landed all 5 without the need for Precision Attack, your damage would be:
5d6 (Crossbow) + 25 (DEX) + 20 (Favored Enemy) + 50 (Sharpshooter) + 5d10 (Menacing Attack) + 2d6 (Sneak Attack).
That should add to an average of 146 points of damage.
You can add an additional 5d6 + 5d10 + 2d6 if the enemy is surprised or an average additional 51 points of damage.
Thoughts?