[Beware! Spoilers!]

On the world map, when I have the airship, there is this big forest to the north-west, almost completely surrounded by mountains. There is a tent in the middle, but when I walk over it, nothing happens. Someone in the city told me that there was some hidden merchant in a forest to the north-west, so I assumed that tent is the place where I would find him. But nothing happens.

So, what's with the tent? And where is this merchant? (Story-wise, I'm at the point where I'm supposed to fly to the mana tree.)

You actually don't enter the tent, you talk to it. Stand over it and hit the spacebar and dialogue should show up. Hope this helps.

Aaah! Why do I have to interact with the tent, while all the other places, I just have to walk into them? What's the reasoning behind this? But anyhow, thanks for the hint.

The reward was a bit meagre, though. I thought I would be able to spend some of my ten thousands of coins.

I don't really know why lol.

Yeah the reward was not that interesting, but it is good to have for the final boss.

Speaking of final boss... how do you actually defeat him? You can't outrun the purple breath and I don't seem to do damage when i hit the center eye after having hacked at the hand eyes long enough. (It just goes "tink".) \:|

Kinda weird how none of my other weapons are available either.

[SPOILER-ish]
After the arms are gone, whack the center eye once, take a run around and see the real weak spot. Hit that about 5 times, he'll change forms (of course) and just hit the blue projectiles back at him 4-5 times to finish him off.

Well, fancy that... i guess that solves that. :)