I went with Halfling Rogue 1st, then 1 level Fighter Archery +2 Ranged, then 1 level of Wizard (Mage Armor and True Strike) to reduce the weight, then the rest Rogue Assassin
depends on starting Dex, I will take Stat increase Dex, then Sharpshooter and then finish off with Alert
to get Advantage with the bow- Sneak using the Halfling ability to hide behind others, corners, bushes, or even go naked with the Bow and Arrows and use the Warlocks Imp to fly with the Halfling invisible (naked to help with the carrying capacity of the Imps) J Crawford in a tweet said that is okay
Halfling shoots and then the Imp goes invisible while carrying the Halfling, anything the Imp carries goes invisible
The last Perk for assassin is at 17th level and the only perk you miss is the Stroke of Luck, but remember the Luck of the Halfling
Stroke of Luck - you hit what you missed or save when you fail - one time during between short or long rests
so with surprise at 20th level - 18th level Assassin 18d6 sneak, 2d6 arrow +3, bow +3 and 12d6 failed save for Purple Worm poison and with a failed save all that is doubled for Death Strike, Could add more damage with the Bracers of Archery +2 damage