These are the bosses encountered during the Story Mode of Super Smash Bros. Armageddon.
The Black Guardian, when faced, will either be aligned with Chattur'gha, Ulyaoth or Xel'lotath. The alignment does not change its attacks nor its strategy.
It cannot move from the center of the stage, but its attacks can easily reach both edges of the stage. It can cast powerful Magickal Attacks, either horizontally (from one end of the stage to the other, requiring the character to jump over it) or vertically (cast directly over top of the character, requiring them to move as soon as the Runes appear). The Chattur'gha and Ulyaoth black Guardians are both capable of using their limbs to try and crush the character or sweep them off the stage; to compensate for this, the Xel'lotath Black Guardian conjures a large hand of raw Magick instead.
The Black Guardian is also capable of Summoning Zombies of its same alignment to attack the player; however, they are slow, and in terms of stamina, about as strong as a Re-dead (except the Chattur'gha Zombie can take a bit more damage).
Although the Guardian can technically be attacked at any time, it does not take very much damage from even the strongest attacks. To defeat it much easier, wait until it casts a Summoning Spell; it will become temporarily fatigued and glow with its Alignment color, and be much more susceptible to attacks.