Vuman (Dex/ Wis) OH Monk, +2 Dex, Alert feat.
S: 10, D: 18, C: 14, I: 8, W 16, Ch: 8; Acrobatics +7, Perception +6, others, AC: 17, Initiative +9, HP 38, Move 40', Saves - Str +3, Dex +7 - Staff, shuriken (darts)
Vuman (Str/Con) Champ Fighter, GWM feat, +2 Str.
S: 18, D: 12, C: 16, I: 8, W: 13, Ch: 8; Athletics +7, Perception +4, others. AC: 18 (full plate), Initiative +1, HP 49, Move 30', Saves -Str +7, Con +6
The Monk could of course simply kite the Fighter to death using his superior movement/ step of the wind to keep at range and throwing Shuriken (monk weapons doing 1d6+4 per hit) and deflecting any return ranged attacks from the Fighter with deflect arrows, but presuming melee:
- Being Dex based (even without Alert), the Monk is more likely to go first.
- Going first, and the Monk can close with the Fighter and attack 4 times (with flurry) spending 1 Ki point to do so.
- Presuming 2 attacks hitting (4 attacks at +7 to hit vs AC 18 = Hit on 11+ - 2 hits), the Monk deals 1d8+4 (17) damage then spends 2 Ki to attempt to Stun.
- Fighter makes 2 saves vs DC 14 at +6 (needing an 8+) If he fails 1 save, it's game over.
- Possible status effect granted if one of the attacks that hit is from Flurry (due to OHM)
Presuming the Fighter is not Stunned, he goes next and likely Action Surges, but does not toggle GWM 'on' seeing as he is likely to miss 4 times if he does so (+2 vs AC 17). With GWM 'off' he likely hits twice (same as the Monk) dealing 4d6+8 re-rolling 1's and 2's (25 damage), and Second Winds for his bonus action.
The Monk gets one more turn to Stun the Fighter (only 2 Ki left so Martial arts only for 3 attacks, and no flurry this turn to re-try the Stun). Again, if he Stuns the Fighter, it's game over.
Those Con saves from the Fighter give him the best possible chance, but I'd still have money on the Monk.
A Battlemaster Fighter with Precise Strike has a much better chance of winning in melee (presuming he is not stunned or wins initiative on round 1) seeing as he can spam all 4 Superiority dice when Action surging to go for the GWM 'on' Power attacks on all 4 swings.
Of course, the Monk could also disarm the fighter (and then use his free object interaction to pick up the Sword, move away and snap it in half via attack an object; AC 19, 10 Hp) with one or more of his attacks, and the Fighter is then in a world of trouble if the Monk succeeds (+7 vs +7 on the disarm).
Even against a creature with excellent Con saves (+6 in this case) I'm not seeing how the Monk is outclassed here.