MAGIC (I) - Battle Magic
Aku is a considerably powerful wizard, although this is something that isn't visited as often as his shape-changing.
This post in particular deals with the forms of magic that Aku has displayed and which would be useful in battle, regardless of the battlefield and its conditions.
The episode Jack and the Lava Monster (S1E10) features a flashback in which a cursed Viking chieftain explains how he was transformed into a rock monster. Essentially, Aku came to his kingdom, destroyed it and to amuse himself used his eyebeams to imprison the warrior inside an unbreakable crystal, which the demon then buried under a mountain so that the magic would keep the prisoner alone with his suffering forever.
After a number of centuries, the Viking developed geokinetic powers and telepathy through sheer force of will and fashioned around his crystal prison a body of rock that he later used to challenge warriors exploring his mountain to duels to the death, in the hopes that one of them would eventually best him and break his curse, allowing him to die a warrior's death. In the end, Jack and his sword were able to break the spell of eternal imprisonment and killed the Viking warrior.
Ringout EyebeamsOriginally Posted by Jack and the Lava Monster
In episode The Samurai Called Jack (S1E2), Aku reveals to a group of aliens that he is guilty for the situation that forced them to request his help. One of the aliens insults Aku who makes the offender disappear with an eyebeam blast. He reveals, however, that rather than disintegrate him, he transported him to a place called "the Pit of Hate" from where he would return once he learned "proper respect". Much later, in Jack versus Demongo, the Soul Collector (S2E10), Aku summoned his minion Demongo from that same place, therefore proving that the place exists.
It is somewhat unclear whether the Pit of Hate is some extra-dimensional nether realm or just some nasty location in Earth where Aku happens to keep both his main minions and folks he wants to give a lesson to. As a consequence, it cannot be stated with certainty whether these eyebeams have a dimensional component or not.
Time PortalOriginally Posted by The Samurai Called Jack
Seen in The Beginning (S1E1) and in every episode opening afterwards. Aku's final move in his first duel against Jack was to "tear open a portal in time" that left the samurai stranded thousands of years into the future.
In order to do this, Aku released energy rings from his mouth which concentrated above Jack, before forming a column of energy around the samurai. After a couple of seconds, the energy column disappeared and Jack along with it.
It is unknown why Aku hasn't used more often this power against his enemies, whether he can make portals to the past too or if he can travel through time on his own.