A boss is typically a test of skill and endurance that you overcome using the methods the game has taught you, usually. There are times however, when what you learned just don't stick or you just don't make those connections and those are times when things can get... Messy...
So my story begins a little more then 10 years ago when the first Pikmin game had recently come out. My family went on our weekly trip to the video store and on that trip Pikmin happened to catch my eye. I picked it out and took it home, and eagerly devoured the game. I was young and inexperienced though, so my first run though the game ended in failure, with me failing to escape from the planet in 30 days and watching Captain Olimar's sad demise. Well I picked myself up and tried again, this time making much better progress, in fact I managed to get all the way to the final boss with just 2 days left. The first day was dedicated to finishing the puzzle to get to him and the second to defeat whatever horrible manner of beast the final boss was.
Well it doesn't take a genius to figure out where I'm going with this story. The way the Emperor Bulblax is supposed to be killed is rather simple, have a yellow Pikmin grab a bomb, throw it in front of him and he'll gobble it up and become stunned, allowing for you to take a good beating to him. It was a simple use of skills the game had taught you and I failed miserably to understand that I was supposed to be using it for this boss.
So how did I beat this thing? In short it was a Pikmin massacre. I threw wave after wave of Pikmin at him, doing barely any damage while he would devour swarms of them with ease. I swear I spent more time running to get more Pikmin then actually fighting this thing. First my red Pikmin ran out, then my yellow, until I was left with only the blue's to take this thing down. I finally managed to reduce the beasts health to 0, but that was only after I had literally watched at least 600 Pikmin being eaten and crushed. It was only years later when I finally went out and bought the game that I found out there was a much easier way to take him down, and that so many of my Pikmin had died in vain.
Another story I want to tell happened much more recently, just back at the end of 2011. The Legend of Zelda: Skyward Sword is the game I'll be talking about and there's going to be some slight spoilers so feel free to skip the next 3 paragraphs if you're worried about that wort of thing.
The final boss of Skyward Sword is great, I really enjoyed everything about it. In fact I think it makes the 2nd spot in my top favourite Legend of Zelda final bosses, only being beaten out by the fact that you shove a sword into Ganon's skull in the Wind Waker final boss. The game heavily relies on moving Link's sword into different positions to do various things, and the final boss, Demise, is defeated though the use of that. During the final part fight you're supposed to raise Link's sword to have it struck by lightning, which you can then use to stun Demise to attack him. And guess what? Again, that went completely over my head.
During the final part Demise tightens up his guard and you're really never given a chance to attack him if you don't use the lightning sword technique. Well try telling that to my past self because I'll be damned if something like that managed to stop me. When I first noticed that he wasn't giving any openings I immediately my invincibility potion, which gave me enough time to ignore his attacks, and do some damage, that wore out, however, and soon I was back to square one, with no way to really attack him. That was until I noticed something though, if I timed an attack just right I could land a hit on him, I would take damage in the process, but it was better then nothing. So on it went, Demise and I trading blows until someone had to give. I had exhausted my potion supply and was on my last 4 hearts when he finally caved though.
It was probably one of the hardest boss fights I had encountered and I felt immensely proud for beating such a difficult boss. It was only until my brother played though the game and we got into talking about the final boss that I found out I had goofed again.
So those are my stories of me constantly messing up, feel free to share your own of bosses or any particular parts in games you did wrong.