Excellent
This is a great game, and people should have no problem winning even on hard (I recommend playing on hard, you get some pretty cool cheats that way, and then you only need to play through once), because I believe that hard in this game is easier than the standard mode in Gnomergeddon.
Here's a small walkthrough for the bosses:
The Mage Master: IMO he's the hardest one because there is no set in stone strategy for him. I recommend picking up one of the powered up weapons earlier in the game to help beat him (throughout the first part of the game I used a sword +15, then later picked up a blade +20 poison), just hit him over and over with a fast hitting weapon to make sure he can't let out any attacks.
Every once in a while he'll summon some fist gnomes or some meteors. The gnomes are no problem, and they'll usually drop a health power up to help you out; the meteors are pretty hard, you can sometimes see the shadows of them before they drop so you can avoid them, but if you are too far up on the screen, they'll blend in with the background.
Anyways, just keep hitting him and you'll win eventually.
The Boar: He's extremely easy once you know how to beat him. Just dodge his first attack and he'll go off the edge of the screen. Now go down to the very bottom and wait to hear him trampling towards you, now go up to the top of the screen and walk either left or right, he'll pass you and throw a spear at you (which you'll have dodged from moving). Pick up the spear and wait for him to come trampling by again, he'll usually appear after 4 or so 'tramples', if you know what I mean. Throw the spear at him to stun him, and now pick up your weapon and start bashing him, you can keep hitting him when the boar rears up, but before his hooves hit the ground, jump up and away. Go back to the bottom of the screen & repeat.
The Freak: Very easy as well, start off by picking up a piece of fruit (make sure you're far enough away so he doesn't OHKO you), throw the fruit at him, now pick up your weapon again immediately and run at him and get in as many hits as possible. He'll jump up, and appear on either side of the stage (the one you're farthest away from) and start charging at you; sprint towards him and then jump over him. Rinse & repeat, he is very simple.
The Gnome King: The second hardest boss, but he's easy as long as he doesn't OHKO you (he will if he jumps on you). Start off by running back into the room you came from and hit some of the jars & barrels, an invincibility power up should be in there, run back and get some free hits in. Once invincibility runs out, keep hitting him for a bit and let him hit you to charge up your rage meter; when you get low on life, go back into the other room and pick up another invincibility (ignore any ordinary healths you see for now), run back and start hitting him. Whenever your rage meter fills, make sure to use it because you're invincible then as well. Just keep doing this over and over again, and make sure not to get jumped on (he doesn't do it that often, so it's not much of a worry); you'll defeat him eventually.
Hopefully that wasn't too long or too boring or too hard to read. Anyways, good luck with the game, remember; play on hard!
P.S. It was a great game.