Any boss from Killzone Liberation, they will make you want to throw your PSP across the room. They will hand you your ass even on normal mode....just like every other normal enemy.
Oh and screw your no RPG rule, even when your maxed gear and level there are still those bosses that will destroy you if you don't have a proper strategy. Magi Master from FF6 isn't hard per say, but if you kill him while he has sufficient magic he will cast ultima on your and there goes your entire party. The only way to kill him is to keep sapping and damaging his magic until he does not have enough to cast ultima and then finish him off.
Elle Kaiser and Diavollo from Crimson Gem saga will rape your party no matter what level or gear if your not careful with your MP and keeping your barrier up. Despite being turn based combat, they will attack 2 or 3 times non stop and just crush your party with some of their attacks.
Ruby and Emerald weapon, do we even need to discuss those guys? Again, your level and gear mean jack shit to them, all that matters is that you actually have a battle plan to beat their cheating asses.