Just here to reiterate what everyone says...
* Play on co-op if you can. Get your partner to concentrate on the big boss
* Concentrate ONLY on the big boss - The others keep spawning.... Don't waste your time unless you need a health potion and they are right beside you (but use your sword).
* Stick to the edge and fall off if you are trapped by the boars charge - it loses you way less health
* USE YOUR SWORD - This kills the small boars and big boar far faster. Yes you can die quickly too, but it's a hell of a lot easier than running around dodging boars and jumping over fire trails. If you are bashing the big boar and little boars come close, bash them as you might get a health potion out of it.
* Upgrade your stuff.. If you are really having difficulty, it might be worth leaving and replaying some other levels to upgrade your stuff before coming back to it. Make sure you at least have all collectibles as they make a huge difference.
I stopped trying to dodge and just mashed sword at the bottom edge of the map and I got it within 10 deaths. I know this sounds bad, but it's a hell of a lot less frustrating because each death is quick. If you dodge and try to pick off the smaller guys first, it will take ages and be a hell of a lot more annoying because of the effort taken to do it. When mashing sword, just make sure you don't get hit by the big boar's charge. Fall off instead... it takes a lot less health off. One charge takes off a huge chunk.
Good Luck... I spent an hour on the fire boss alone.