The Barbarian goes into Rage and attacks. Far simpler to do. Most of the time he is going to outdamage you against legendary boss foes at your level.
And the goalposts keeps moving...
No setup, no buffs, no allies, no magic items, and I lose concentration every round. But I will at least assume we can both get into melee range.
Beserker Barbarian with GWM always getting a bonus action and reaction attack with a maul.
2d6+7Str+4Rage = 18 * 4 = 72
+12 GWM + 4d6 Berserker =102
102 / .85 = 86.7 damage per round.
Assuming a d10 weapon for easy calculations. 9 attacks per turn (10 on turn 1).
1d10+5+10d8 = 55.5 damage per attack.
9 *55.5 * 50% = 249.75 damage per round.
CME is doing 288% the damage if the barbarian.
I could lose concentration and quicken CME (7th) and still damage the barbarian. Then I could concentration a third time and quicken CME (6th) and still out damage the barbarian.
Quicken CME 5th + EB
5d10+25+20d8 = 145* 50% accuracy
= 72.5
Not quite. I either need to either make my concentration check, or cast at 6th level to out damage the barbarian.