It would be very drastic indeed and honestly wouldn't even know how it would end up working, which would make me feel unsure about how I'm playing him and lead to dropping likely within two months, and that's what I want to avoid. I would prefer a penalty that is annoying, but something he can work with. He's also not likely to request removal on his own, so I have to consider something that will be fine to remain there forever or annoy him to the point that he'd be willing to ask.

But as far as plot, I'm figuring that at runner stage, Leonhardt still ends up pretty much around where he is normally. Dropped down to human and then boosted back up basically seems like he'd be around his normal. Of course, normal for him is pretty damn strong and fast.

Honestly, anyone would have the best chance once it advances further and he turns to a shambler because he won't have his speed and skill with a sword left.

Though another potential idea, perhaps getting Saleh into a situation where he's cornered and is left with the option of either killing Leonhardt or becoming infected himself. Desperation can definitely lead to immense strength. In either stage Leonhardt won't have his ability to create cunning strategy and dodge nearly so well so that might work. At the very least, it may be enough to gain an advantage.