I can add a few cents here.
Shinryuu (FFV): Locked inside a chest deep in the final dungeon, Shinryuu is a massive water dragon that I never challenged more than once. (Thankfully, I didn't have to beat him; he's optional.) The battle only lasted about 30 seconds; after I smacked him a couple of times, he hit me with Tidal Wave, which dealt around 7000 damage to my entire party. None of my characters had more than 4500 HP. You do the math. Needless to say, the next time around I completely skipped the thing and beat the game without the help of whatever was inside that treasure chest.
Dullahan (Golden Sun: The Lost Age): Dullahan is nearly impossible to beat without the right strategy. He attacks three times per turn and can do things like hit you with summon attacks and completely destroy your stats.
Evil Wall (FFIV): I HATED THIS STUPID THING. It starts the battle at the far left of the screen. Every few seconds, it moves forward a little, slowly approaching your side of the field. Once it's close enough, it proceeds to Crush your characters, wiping them out one by one. Once this starts, you're doomed; it's impossible to revive people as quickly as they're falling. Your only chance is to blast it with everything you've got and hope it dies before it reaches you.
What's wrong with explaining? If no one says anything more than "Oh, I thought this guy was tough," then there's no discussion. Plus, people who haven't fought these bosses might like to know what they're up against should they decide to play these games.
