Rayman 2's final boss goes something like this: You're riding a missile. There are two nearby dead-end tunnels that are so small that doing a U-turn at the end is very difficult, but you will need to because they have a powerup that lets you shoot the boss. Also, the boss will occasionally shoot homing rockets at you. You need to shoot the boss's two arms, which are pressed against two pipes to keep him from falling into the massive body of lava that's under you both. Each time you make him fall, he will lose some health, flail around a bit and then bring himself back up.
Now see, what made this long for me was that I didn't realize while he was flailing you could keep shooting him. I just saw the little health decrease and figured that's all I would accomplish. So I kept going through those small side-tunnels to get more and more powerups to deal infinitesimally small amounts of damage to him. AND as his health goes below half he starts doing this thing to make the lava rise, causing the U-turns in the tunnels to be even more deadly.
The kicker? No health. If you hit the wall, get hit by his missiles, anything; YOU DIE AND HAVE TO RESTART. Maybe that's the thing that made it take the longest.