This table shows the health and attack by dungeon difficulty of every boss (although at Hard difficulty, it shows "reference" statistics -- boss statistics vary wildly in Hard dungeons; for example, Grimm's Grotto significantly boosts health and damage, Doubledoom slightly depresses them, and Hexx Ruins, though nominally Hard, uses the Normal statistics):
None of the forms are particularly tough, but a hybrid of physical and magical damage will have the easiest time. Only the final form awards XP. It's recommended to fight each form one at a time, making sure to recharge between each with either a level up or remaining blackspace, as Anoobis can only regen to the top of his current form. Trying to do all three as if he were one singular monster makes the fight significantly tougher.
Note: Anoobis provides no experience or gold when killed in his first and second form.
The berserker boss is Bjorn, and he adds magic resistance to the normal berserking special of his lesser kin. In effect he's a toned down version of Bleaty. As a result you can't count on BURNDAYRAZ to pound him into submission, but he's still held back by low hit points and general fragility. Any method of a prolonged non-fireball damage spike can take him down easily.