A Blade pact Posilock 11/Sor 6 can use Greenflame blade with a greatsword and GWM to deal 2d6+3d8+10+strmod+3xchamod (+hex?)
Lifedrinker requires Warlock
12th instead of 11th, it's out-of-sync for some reason.
And it comes out to 56/rnd to the main target (if 29 str & 20 cha), which is still below Barbarian/Fighter figures so it could use some more help.
I'm honestly more interested in the Ranger, can you combine them?
Class: Skirmish Ranger 1 / Deep Ranger 3 / Assassin Rogue 3 / Battle Master Fighter 11 / Anything 2
Feats: Alert, Martial Adapt.
Items: Oathbow*, +3 arrows, Bracers of Archery*, Ioun Stone(mastery)*.
Actions: Bonus to cast Hex, Attack (ambuscade) + Ud Scout, Attack, Action Surge->Attack.
Attack: 1d20, +7 proficiency, +5 dex, +2 archery style, +3 magic, +3 advantage's odd tweak. & +1d10 precision attack x6.
Damage: 1d8, +3 magic, +5 dex, +3d6 oath, +2 bracers, +1d6 hex. & +2d6 sa x2.So vs AC 25 (aka Tiamat),
1d20+1d10+20 is an 99.75% hit (double 1s), deals 47 (2d8+8d6+10), six attacks is 281.295 damage.
The remaining four are done at 1d20+20 (75%) for +141, then don't forget SA's +28 (2d6*2*2).
For a total of 450+ in a single round as a solo build with two levels to spare.
"Do you like Rangers yet?" ~WotC on their bow fetish.