Actually, the battle with Sin in FFX. I mean, a bunch of young adults and teenagers are supposed to defeat the local eldritch horror by swinging various swords, staffs and rubber balls of doom. (Plus the occasional fireball, which, while being impressive looking and undoubtably fatal to a normal-sized foe, is at best going to leave a nasty scratch on something thats the size of a cruise ship.) Its around 200 times the size of them and is armoured with scales!
I'd say in terms of difficulty clearing Streets of rage on Very hard... but thats mainly because I suck at it. I can't even clear easy!