Well, I'm not sure the HSD characters are quite fast enough to react to the bell (although they obviously react faster than him) and cover 100 feet before he can think 10/10 times, speed of thought being 30/ms for nonspeedsters, and I don't recall slicing time ever taking anything else, as we've seen it used reflexively. Furthermore, Lu Tze has save points, so if he does get going, killing him might not actually kill him. My point with the lightning bolt thing was mainly that he can slice time deep enough right off the bat that a lightning bolt becomes something he can react to and catch up with. I'll give you the durability thing though, only time slicing + deja fu means he can do just about anything he wants to them if he isn't killed immediately, so he might be able to manage something. But no, you're right, he probably can't hurt the majority of them even if he can strike them any way he wants any place he wants. Maybe death by kancho? He could at least make them really uncomfortable....He has no feats to suggest that he could think before Shigure cuts him in half for example.
Anyway yeah, I don't see anything short of phasing or something like Sue Storm literally putting a TK bubble in his brain or heart straight up bypassing Ryoga's durability. Even constricting blood vessels is a little bit iffy if they don't have feats like Manchester Black's.
Edit: And yeah, I'd really like to know which character in HSD did this, 'cause straight up chi magic seems really out of character for HSD. I can't recall any technique where they haven't given a clear physical description of what was happening except maybe for Hayato's memory-wipe thing, but that was Hayato, and he gets away with anything.