Aten-Ra is the ruler of Armana. They are also the Guardian of the Dark Lord's Mausoleum.
This boss is massive and spawns a ton of enemies. Thankfully, they don't take down very much HP.
Aten-Ra spreads out a bunch of arms from his body, which you should destroy. When all the arms are destroyed, he smashes the ground with his huge body. Stay on the sides when this happens, because the smash can OHKO you!
After the smash attack, he drops his guard and starts attacking you: This is the best (only?) time for you to attack back. Repeat the process until he's dead: The enemies he spawns, though many, don't hurt very much.