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.