Normally you frown on this sort of thing. But sometimes, character discrepancies come up, or a PC gets controlled by the BBEG and you want a challenging fight. But herein lies the problem —! OH S*** THE PC HAS SUCH LOW HEALTH. And all the party members can explode you if you get initiative. Dumb and uninteresting.. right? I think so. Especially if the controlled PC is supposed to be a boss!
For duels, there's an easy way to solve it. One on one duels (tense and dramatic) can simply have the PCs multiply their HP by 5. Otherwise, halving damage dealt to PCs in PC on PC conflict might be a way to mitigate the overwhelming HP to Damage ratio.
Even average level 30 PCs have little more than 500 HP, but they're likely doing much more than that in a single hit(at least tech-wise). You don't want PCs accidentally ganking each other in a drop of a hat, especially out of combat (if you allow it) where they just rip off an attack at a party member to stop him from doing something (dumb or otherwise)… and KO them. Trying to figure out a concrete way of doing this should be looked over. Halving damage to begin with sounds like the best bet, again. (Even for characters under confusion, attacking party members I feel)