Balance issues: (on hard difficulty)
The Beerholder and Minocow need buffs, I do not think that competent players lose against these bosses ever. I haven't played them so I'm not sure exactly what they need. But I would say that two main things need to be considered, their ability to inflict damage, and their ability to interrupt player resurrection. They do not seem to have the ability to hurt players who are avoiding them, such as a rogue or mage who is playing it at a safe distance. They have a massive dead zone at range where none of their abilities are remotely accurate and they cannot force players out of the dead zone for any extended period of time. As for player resurrection, they cannot stop an invulnerable resurrection, they cannot move a player affected by the ray of awesome. Since they lack that ability, it is nearly impossible for them to keep a good group of players dead.
The Elder Dragon could use some buffs as well. For starters, the ranged attack needs to refocus the camera so you can follow players outside of your normal camera range. Melee attacks could use a buff, perhaps a range buff on the jumping slam attack.
I believe the lumber tree is the only boss that enjoys having minions not drop helpful items to the heroes. Why should the other heroes get food items or life ups on fights other than this one? I can't think of a good reason. Why should life ups drop during boss fights at all? There doesn't seem to be a good reason. It hinders bosses like the hippo who's consistent damage is rather poor.
Wizards: The weakest creature that a DM can employ. As a possessed creature, their animation time for an attack is absurdly long and their stun isn't long enough to even guarantee a hit from point blank range. As a typical monster, they are one of the easiest creatures in the game to backstab as a rogue, they die quickly to aoe damage, and they do poor damage when they actually reach an opponent. Their one redeeming factor is that they have a stun, otherwise there would be absolutely no reason to use them, this lowers their value to a support role. Suggested changes: Reduce animation time while possessed for their attack. Increase the range of the stun while possessed. Increase stun duration while possessed. Increase their combat value as a typical monster.
Orc Berserker: Their enrage ability is both their strength and their weakness. When the party of heroes has the tools to deal with orc berserkers, (damage output, usually from range) orc berserkers can find themselves stunlocked from their own rage, even the elite version. When the party of heroes lacks the tools, (insuffiecent damage output) they can use their absurd attack speed to clobber slow party members such as the warrior or mage, especially when it is an elite and/or possessed. Please place a cooldown on the enrage effect, increase the health amount that triggers enrage on non elite versions or change the mechanic behind enraging, lower the bonus attack speed and movement speed gained from it, and allow a possessed one to choose when to enrage. That should balance out the possible scenarios a bit.
Fat Phoenix: It is incredibly hard to hit anyone with it's attack, as it goes at a set speed, in a set direction, to a set location. Please allow players to end their movement at any time during the movement animation to attack the position the phoenix is currently hovering over. You might also consider giving them another ability while fattened up.
Spider: Do not allow the spider to stun an invulnerable player, excluding evasive maneuver's invulnerability if you decide to keep that aspect of it. I can stun a player who just recently revived, while they either can't move, or haven't reacted to their revival yet, and kill them off again.
Monster circle generates 8 minions regardless of what type of minion you are summoning. It should vary between minions. For ducks, 4 big ducks and 4 small ducks. For small minions like wizards, double the amount of wizards, perhaps adding layers of the circle instead of forcing all minions in the same circle.
The evasive maneuvers perk is simply absurd. Please nerf some aspect about it. Perhaps lowering the damage reduction or reducing the amount of rolls they can do before getting exhausted to two or three while they have the perk.
Improve consistency of the warrior's shield strength between difficulties. I don't know what the numbers should be, but warriors in normal and easy are nearly indestructible with their shield and become outclassed by the other classes in hard, (DM mode) harder and impossible. (I assume, I haven't really played those difficulties as or with a warrior)
Normalize shield break damage. When breaking a warrior's shield, they take the amount of damage the attack would have dealt, (as far as I can tell) regardless of how much damage it took to break their shield. Please normalize the damage to around 25% of their max hp (changed by damage modifiers, such as the difficulty setting and the curse trap) so a DM wont be punished for not using a strong attack as a final blow to break the shield, and that warriors will experience consistency in their shield break damage.
The healing supplies potion provides too much healing per potion use. As a comparison, the mage has two healing spells, one that heals half way and rezs, and another that another that heals to full I believe and also grants invulnerability for a few seconds, and the rogue has the bomb which heals to full, whereas the healing supplies grants 6 flasks which heal a large portion of life. (~70% as far as I can tell, may as well be a 100% heal) Please reduce the potion count to 5.