The first fish thing you fight in FF10 with only Tidus.
Having only Tidus, it was a huge ordeal imho.

Also, I believe Gargos in Killer Instinct Gold was ridiculously evil since he could gain his life back and the only way you could really beat him is to finish him off with a combo that tossed him into the air, which he always seem to combo break his way out of.
*shakes fist*