Designing a boss-type opponent, would like advice

[MonkeySage]

I currently have most of the party competing in a sort of brawl tournament. It's one on one fisticuffs and no magic, so I would like to be able to create an opponent that my party would be able to defeat with some difficulty, assuming that whoever fought her started the fight with full health.

Most of the party are monstrous humanoids, sans racial hit die. There level adjustments are +1, but at class level 4 the strongest member of the party(a hexblade with a homebrew race) has 36 hp and 23 strength.

The boss character I would like to make is a Sohei, sort of like the eastern not so equivalent of a paladin. They cast like a paladin but don't have access to all of the spells a paladin has, and they can't turn. Instead they can Ki frenzy, which boosts strength and dex sort of like barbarian rage.

So the members of our group she is most likely to fight:

Half oni Hexblade(LA +1), Str +4, Con +2, Dex -2, Wis -2(no hand to hand training) Class Hit dice: 4d10
Vanara shaman(adjusted LA +1), Str -2, Int +2, Wis +2(as a shaman, hand to hand skills run close to that of a monk): Class Hit Dice: 4d6
Lizardfolk monk(no racial hit die, LA +1): See monster manual, but take way the racial hit die. Class hit dice: 4d8

Any advice?

[ArcturusV]

Hmm, well, the Oni should be a chump not being trained for unarmed combat. Everything he does will provoke AoOs which can shut him down pretty hard. So I'm not too worried about that.

So it's mostly a case of Monk and Wannabe Monk versus a single champion. Sohei is... okay. I like the class. Probably not optimal if you're looking for a tough champion.

Depending on how far you're willing to reach? A crit fishing unarmed combat build with a little luck can really mess people up. So you look at feats like Roundabout Kick (Free attack at the same bonuses when you crit, if you crit your kick it seems to give another kick...), Improved Combat Martial Arts (Doubles unarmed crit range), Nerve Pinch (Forces Fort saves vs Paralyzing on unarmed crits), Falling Star Strike (Blinding on a Crit)... hmm... I'm sure there's some other feats that could expand the Unarmed Combat Crit Range... but I can't think of them off the top of my head.

Of course you could try shenanigans like being a monk (Or other class that says unarmed strikes can count as a manufactured weapon), amputate your hand, replace from the wrist down with some Masterwork item, enchant the hell out of it and give it the Keen Property, etc. Call it good.

[MonkeySage]

Looking around for suggestions, I got one i kinda like. The party have seen her wearing armor and carrying a naginata... But one person suggested I use the armor to handicap her as a monk, and excuse her naginata carrying habit as it being an ancestral weapon she doesn't actually use in fights, or a ceremonial tool at best.

When she fights the monk, she could just remove her armor and prove that she's "not left handed".