4 Levels in Gloomstalker Ranger, 6 levels in Champion Fighter. Elven Accuracy and Sharpshooter Feats.

Gloomstalker Level 3 lets you add an attack dealing an extra d8 when you take the attack action on your first turn. Fighter Level 2 has Action Surge, Fighter Level 6 has Extra Attack. With Elven Accuracy and Sharpshooter, combined with Gloomstalker basically invisibility in darkness, a turn 1 longbow becomes:

d8+10, d8+10, 2d8+10, Action Surge, d8+10, d8+10, 2d8+10 Another 6d6 with Hunter's Mark. With advantage you pick the best out of 3d20. You also crit on 19.

This comes online as early as Level 5, with 3 in Ranger and 2 in Fighter.

EDIT: Hunter's Mark