I do think Viewtiful Joe is hard, but it is no where near the hardest side-scroller I've ever played (that honor goes to Skullmonkeys)

As for the Fire Leo, this is how I beat him:

At first when he's making the rocks fall, just fast forward and punch them until you're on fire. When you're on fire, his fire attacks cannot hurt you.

I don't clearly remember what to do next but I'm more than positive you can run up to him and start attacking him at this point while you're still on fire.

Next he will start spinning and try to punch you.

What the game WANTS you do to here is dodge his punches either pressing up or down until he gets dizzy, then they want you to go in there and unleash.

What I did at this point was go slow mo, zoom, and do the spinning kick while he was in the tornado. You will be able to hit him with the zoom spin kick, and you won't take damage from his punches. The kick won't take off very much of his life at all, but it adds up. Keep the spin kick going until your VFX is about depleted, and then go out of the slow mo and dodge his punches like you're supposed to until your VFX is full again. When your VFX is full again, go back into slow mo, and zoom kick him again until you're empty.

At this point he's going to start his pattern all over again, so just repeat until he's dead. When he's almost dead he's gonna start unleashing on you like a mother, and when he's charging on all fours he'll be fast as hell. Probably too fast for you to outrun. What I did when he started his really fast charge was hide under the platform. I think I took a bit of damage, but I took less than he would have done to me.

All in all, Fire Leo is the hardest boss in the game. If you can beat him, then the final boss will pose no problem for you.