LogFAQs > #974049497

Onimusha 2 (and possibly other games in the series, 2 was the only one I remember having it), had "Issen mode", which only allowed you to damage/kill enemies with super hard to pull off counter attacks which were pretty much instant kills for monsters and bosses. For context, I think in my average playthough I was lucky to pull the move off 3 or 4 times. Beating Issen mode would require you to do it hundreds of times.

Shinobi and Nightshade on the PS2 also had a pretty cool combo system which, if done correctly, one-hit kills most bosses. It requires you to slash and kill enemies, building up power with each kill until after a few your slashes can essentially one-hit kill them. Eventually, you slash the boss and do tons of damage, which in most cases kills them instantly. Really fun when you can pull it off.