Half life? Hard? Hahaha... Really... or is it HL2? Haven't played that one.
Anyways, I found the bosses were the most fun parts of the game... That is, after I learnt their respective patterns.
The castlevania boss was intense. It was so challenging it made me play it again, and again. (Because I died)
The mecha birdo battle was pretty well thought, what with the jumping and the epic music.
Mike tyson was kinda hard for me because I didn't know he threw fire. I just shot at him. Not my fav boss.
Zangief ridley. It's bugged, so pretty much he does stuff he's not intended to.
Man, do I hate the clown copter. It's the boss I like the least. The thing I found good was how it popped unrelated characters.
Mother brain. Heh, dodging those zillion of bullets wasn't easy, but it made the boss battle different each time, since they alawys pointed at you.
Dragon boss? I hate the jumping part so I skip it. The rest is Excellent®, specially when you jump between the giant blobs. (A little suggestion, couldn't Kayin make it so the dragon selected, randomly, some predefined combinations for the blob throwing part? To keep it surprising)
The guy...
Man did that took me the time! It was severely frustrating. But then, when I beat him, I discovered how easy he was.
I was all like "All I had to do was jump here and here, and when he stops shooting, here!".
In all honesty, the bosses were some of the most fun parts of the game. Here's hoping Kayin implements moar.