This isn't a problem that a mod can fix. Rather, it's a problem that a mod has introduced.
Brawls are supposed to be magic free. Or rather, you're not supposed to use any sort of magic on your opponent. When you do, the brawl becomes regular combat, and your opponent will fight you to the death. But most brawl characters are essential, so they cannot die; they enter bleedout, then go hostile again when they recover.
What's likely happening is that one of your mods has applied a magical effect to the player that acts as a "cloak" of sorts. Wet & Cold is one example, I believe. Can't tell you what might have caused it, though, without seeing a BOSS log. In any case, your opponent is seeing this effect as hostile magic, and wants to kill you for it. There's a modders resource out there called the Brawl Bug Patch that you can install separately, which fixes this issue. It's designed to be incorporated into problem mods, but can be used as a standalone mod in itself I believe as well.
Tsun is different though. That's not a brawl, that's a fight. He will kill you if it means keeping an unworthy soul out of Shor's Hall. Doesn't matter who you are - if you can't beat him, you don't have a hope of defeating Alduin anyway. That's not a bug, it's just how the game works. What do you want him to do - let you in even though he beat you, as reward for being a noble loser or something? Let you try again until you succeed because the Dragonborn has low self esteem? What's the point of the fight in the first place, then?
Edited by Hyacathusarullistad, 11 May 2014 - 12:46 PM.