Frankenstein and Igor, Castlevania

This horrific duo was the first boss battle that ever stopped me cold in the old Nintendo Entertainment System days.  The original Castlevania was a hell of a challenge after the battle against Medusa, but by the time I reached the end of the fifth level and met Frankenstein and Igor at the end of a long tunnel loaded with Bone Dragons, I'd met my match.  Frank himself is an easy target due to his large size; it was Igor that was the problem.  He leapt around the room spastically and tossed fireballs nonstop, meaning the fights lasted only a few seconds.  It was nearly ten years later that I was able to finally defeat them and move on to the next level.

Womack Spider, Plok

Plok from the Super NES era is a very colorful and animated video game, but the happy visuals cover a steep difficulty level.  Nearly three-fourths of the way through the game Plok faces off against the Womack Spider, a massive arachnid.  On his own he's probably not that big of a threat: move, move, shoot; move, move, shoot.  The challenge is that by the time I reach he spider's layer, I'm out of continues and low on lives.  It's an easy finish for Plok, and then it's Game Over.

Lavos, Chrono Trigger

RPGs are known for difficult boss battles, and the Super NES's Chrono Trigger is no exception.  The alien turtle-like Lavos can be engaged in a number of ways, but starting the battle from his outer shell is one nasty encounter.  His first form is deadly enough due to various attacks with his powerful spines and nasty death breath, and by the time I was able to punch through to the lifeform inside the shell, my team was always weary and weak.  The second of Lavos's three forms, the heart, typically finishes off the heroes because I'm out of regenerative abilities and cannot keep up with the many attacks.  The final form, the core, was rarely seen in my house back in the day.

Omega Pirate, Metroid Prime

Metroid Prime is another game loaded with challenging bosses, but it's the Omega Pirate near the end of the game that has my progress halted.  This Phazon-enhanced beast is just too quick and too powerful, turning invisible and unleashing other enemies as a distraction.  I inflict a decent amount of damage, but it's never enough; heroine Samus Aran runs out of energy and lets out that horrible scream before falling down dead.  Metroid Prime becomes more frustration than fun at this point, and going back and trying again for the umpteenth time isn't very appealing.

El Gigante, Resident Evil 4

El Gigante is the most difficult boss I've faced in video gaming over the years, and it's not because he's necessarily so strong or difficult to defeat, but because he's a complete psychological mind trick.  Gigante makes his entrance fairly early in the game by smashing and crushing the very people who released him in the first place.  His massive size is impressive enough, but how does one deal with a boss who kills its own underlings/supporters?  Hero Leon Kennedy is armed with only a few meager weapons at this point, and when Gigante made his appearance I was stunned.  How could I ever hope to defeat this behemoth armed with a mere handgun?  How could I counter Gigante's habit of ripping trees out of the ground and swinging them?  And what's with the tentacle that rips out of Gigante's spine?  Defeating Gigante wasn't so bad in the end, but the process of psyching oneself up and getting past the intimidation before engaging him takes some planning and thought.  That's the challenge behind El Gigante and, overall, Resident Evil 4 itself.