Rage of the Gladiator/YMMV

• That One Boss: Master Yee for the second time is the best candidate, as he has a very quick attack that has instant execution and no warning, basically requiring you to have your shield up for the whole fight (something no other fight, not even the ones after him, requires you to do).
• That One Attack: Every boss has at least one attack that will mess you up the first time you encounter it, especially in Challenge Mode. Master Yee's very quick staff attack, as mentioned above, will frustrate you to no end until you figure out to have your shield up for the whole fight (which had never been necessary until that point and won't ever be necessary again, except in Challenge Mode).