A lightsaber duel with a severely wounded, mentally unstable, overconfident and under trained Dark Side warrior
The physical toughness of these characters is not in question, it is their emotional maturity.
No, it’s the physical toughness though emotional maturity does play a part in why he loses because he could have killed Finn within literally 8 seconds. Yet chose to prolong the fight and in turn Finn got a lucky strike on his arm which contributed to his loss to Rey.