The One Shotter (5e Optimized Character Build)
From D&D Wiki
This is a level 20 build, and has some awkward stages along the way, so play it at your own risk, it is more of a theorycraft.
Fighter level 2, Paladin 6(conquest), Hexblade Warlock pact of the blade 12:
Fighting style great weapon. instead of feats boost your charisma to 20, leaving one feat if you want great weapon master or something.
For eldritch Evocations take Eldritch smite, Improved Pact weapon (because this level 20 character stepped out of the void and has no magic items), Lifedrinker, and 2 others
When you run into Asmodeus, or, you know, whoever...
Round 1, approach - put your hex on him and your hexblades curse (you can use 2 bonus actions in a turn if you don't use your action) (EDIT: You can't 2 bonus actions in a turn.) Round 2, Let it all out - Action surge for 4 greatsword attacks with advantage that crit on a 19 or 20. Use your channel divinity to give the first one a +10 to hit. 1st attack is +22 to hit, drop an eldritch smite (6d8 force) and a paladin smite on top of that (level 2 if it crits, level 1 otherwise) (asmodeus is a fiend, so you get that extra d8) 2nd - 4th attacks are +12 to hit and have advantage (eldritch smite knocks prone) with 2 more eldritch smites and 3 more divine smites (2 of which are level 2)(EDIT: You can't use multiple Eldritch Smites on the same turn)
4x greatsword 2d6 + 6(hexblades curse) + 5(hexblade adds charisma) + 5(lifedrinker) +1 (improved pact weapon) (slashing) 4x 1d6 (hex) necrotic 2x level 2 divine smite (3d8 radiant or 4d8vs fiends or undead) 2x level 1 divine smite (2d8 radiant or 3d8vs fiends or undead) 3x level 5 eldritch smite (6d8 force)
In addition you have a 10% chance of critting on the first swing, and a 19% chance of critting on each subsequent swing, with a 53% chance of critting at least once during the process. If you do crit, you get an extra attack out of great weapon master on your bonus action. Too bad you only have level 1 divine smites left.
You probably shouldn't use great weapons master attacks (-5 to hit, +10 to damage), since a miss loses you a minimum of 3d6 + 3d8 + 17 damage (there is one swing when you only drop a level 1 divine smite, 2 if you crit), but you could get an extra 40 damage out of that, too.
If all the attacks hit and you roll 1 crit, rolling average damage on all the dice, you deal 18d6 + 46d8 + 85 => 355 points of damage. Not enough to kill Asmodeus, he wins.
If all the attacks crit and you roll max damage, you deal 30d6 + 70d8 + 85 => 825 points of damage. Congratulations, you are the new lord of the 9 hells. You can probably afford a magic weapon now.