Nullaeus Nemesis Boss Guide
She doesn't even need to be a particularly high level, though it will make the fight that much easier.
The secret is really no secret at all. It's Valeera. Turns out, our seasonal Delve companion is quite capable, especially once she's gotten some levels under her belt. As a damage dealer she nails every kick with Dagger to the Throat, handles the intermission adds all by herself, and can face tank Nullaeus' attacks with a little bit of support. She'll even automatically Assassinate the boss at 5% health!
Healers have the easiest time with this, as they can sit comfortably at range and let Valeera take the role of DPS and tank. As long as they stay below 130% threat, Nullaeus will never turn his attention toward the player, and if he does, they can just run away until he loses interest again.
Ranged players can follow a similar strategy, though they will have to contend with Nullaeus' ire. In this case, the strategy is the same - just run away from him. The majority of his damage comes from auto-attacks, which can't hit you if he can't catch you. It's not as hassle free as it is for healers, but the combination of your damage plus Valeera's can make very short work of the boss. Hunters can also use their pet to tank, making the fight about as free as can be.
Melee damage dealers and tanks have the hardest time here, since they can't simply kite the boss and still contribute to the fight, forcing them into melee. Those with strong self sustain can try keeping her on as a damage dealer to burn the boss quicker, but using her as a Healer may be easier, since she will immediately dispel Devouring Essence... though you will now be responsible for kicking Emptiness of the Void!
In fact, the only thing she's not particularly good at is being an actual tank, holding less threat than her damage dealer specialization and still relying on players to keep her alive.
Loom has a wonderful guide explaining the fight for healers.
