I find the game pretty cool, except for the boss battles...
I literally spend days trying to get past them even when I know what to do.
Is there any way to add health or something?

Yes, you can find plenty of health upgrades. For a full list, look here:
http://jayisgames.com/archives/2011/11/treasure_adventure_game.php#walkthrough
I've gotta admit, this is one of those games that makes me want to smash something, so I completely understand your frustration with the boss battles.

The game definitely has some big difficulty spikes. The biggest thing for the first rock boss is to make sure to get a heart container beforehand, as all of his attacks do 2 damage so that one container adds quite a bit of survivability. There's one available on Cologel island under all the quicksand. Also, when he's doing the "hands smash together" thing, jump after the hands have hit the ground, RIGHT as they start to turn, or you'll still take damage.

The other boss I had trouble with was the sewer boss, until I discovered that if you stand very close to him, he won't shoot missiles at you. Or if he does, they'll go right over your head. His missiles also won't hit you if you're hanging from the ceiling. Standing close to him (but not too close, or he'll hit you when he submerges) took that fight from "agonizingly hard" to "piece of cake" for me.

Most of the other bosses can be fought after you've acquired a whole bunch of heart containers, which makes them a lot easier. Also, as someone who grew up playing stuff like Mario and Battletoads, this sort of game is fun, and refreshingly difficult, but it was never really controller-smashing difficult, at least not for me. But that extra layer of frustration just adds another layer of satisfaction once you actually do win!

I was having the hardest time with the rock boss. For me a big part of the problem was the keyboard control. I bought an adapter so I could use my Xbox controller on my PC and it made it much much easier. It still took a few tries but it wasn't so *head desk* inducing.

I think the default control scheme isn't very good. UP actually means "activate" 85% of the time, while Jump just moves you upward, so I made jump into the up arrow key. Made my life a lot easier. I made Attack into Ctrl and Action into Space, and UP/Activate the spacebar. I found that to be immensely easier to use than the default configuration.

However, for the final boss and the computer boss, I had to switch UP and Jump back, because in those cases, up actually DOES move you up, and moving up with the spacebar just doesn't work well.

Take it easy at the beginning. Remember - you have your game conveniently saved just before the battle so you can allow yourself... to die. If you just chaotically try to avoid the enemy you will probably just die. And than again and again and again. The enemy has usually two or three types of attack. Be sure that you know EXACTLY how to avoid them. You will probably learn the hard way and die at the first encounter but then you will easily beat him. Just as bevinator wrote about the sewer boss. Watch and learn - that's the key.

I think the difficulty is just right, especially with the boss battles. Even if you know how to beat the they're still a huge challenge, you are actually forced to learn attack patterns which isn't a bad thing at all. Once you get all the hearts it becomes much more forgiving in the boss battles but still hard.

What frustrates me more is the precise platforming sections where if you miss a jump and fall you have to go back and start the platforming section all over again. if you played a lot of this game, you know exactly what I am talking about.

Too true! I do know EXACTLY what are you talking about.

And I must confess - I cheated a bit at one point to avoid this problem: after acquiring the wind spell in the animal village you can save your game. And those enhanced jumps were very difficult to me so whenever I fall I just quited the game and loaded it back because I was in mood for going back to the village the hard way. I know - I will burn in hell for this ;)

That's funny, I am at that exact point looking for the flash light. Then I am going for treasures 8-12

So you have a good chance to burn in hell as well if you follow my lead. It would be nice to meet a fellow gamer though ;)

I've finished the game this morning so I may tell you that getting the flashlight was the most frustrating part. Perhaps it was my bad - when I wanted to make the enhanced jump I released the wind horizontally instead and was blown away to the ocean.

yeah i did the same thing as you and got the flashlight plus treasures 8 and 9. It was worth it but gamer hell sounds scary they make you play the worst game you ever played ever forever and ever.

In all seriousness I don't think it is cheating, who wants to run allllllllllllllll the way back to the animal village ten times because of some of the trickiest platforming section in the game, not me nor you or anyone with a sane mind.

Sorry for the double post but I just finished it and damn that was a good game, Thanks GOG for offering it. in the future I think I should buy the soundtrack because it would support the dev and the music is just that damn good.

The flashlight part was very very tricky because of how strangely the bottle works. However, if you accidentally press over too early, hitting the opposite direction will make you fall straight down, so you can land on the platform below and try it again without having to restart/reload from the village. It's also tricky because you have to use the bottle in midair... which you need to do in several other places too. Probably the most frustrating part of the game for me, as well.

The correct sequence is to make sure the bottle is pointing down, then jump, then activate the bottle, THEN press over, but make sure you press down again before you activate the bottle again. While you have the wind spell over you, jumping will fill the bottle, so at least you don't have to worry about that. Jump-attack-over-down-repeat. Someone described it on another site as "a miracle of platforming," which I have to agree with, but I've gotten a lot better about it. There's a Let's Play video on youtube (part 28 I think) that illustrates proper wind-bottle use which I found to be very helpful.

Now you've scared me! That would probably mean Frank Herbert's Dune for me and man! I cannot even stand thinking about it!

And just another thought about bosses in TAG: The most difficult ones for me were the one guarding the first treasure and the sewer worm. Therefore it may be concluded that having your health boosted really matters! So give it a try and find some hearts.

I'm still at the part where you get the paper hat, those damn spikes keep killing me xD