Here he one blast kills He-Man; who was comparable to Pre-Crisis Superman at the time, besides that he gets stabbed through the chest, converted into energy and fed into He-Man by He-Man's power sword to heal He-Man from death and he shows up later on in other issues. So being effectively destroyed, eaten and digested can't keep him down for long.
Here he blasts a superman flying at him with his power sword, sending him flying unable to stop himself, he also managed to do it before superman gained any ground after dropping off beastman and starts flying towards him. Not a sign of super reflexes but decent ones. Though the magical nature of the blast is at least partially responsible for the effect.
Here he dodges He-Man's axe and does a spell that controls Superman; a spell linked to will power, when Skeletor gets briefly distracted Superman is able to reclaim his body again. So Skeletor's will is slightly better than Superman's, a being who has one of the strongest wills there is. It's also the fight between Superman and He-Man, a strength feat for skeletor as he breaks out of his stone prison and a teleporting feat for skeletor, a four in one kind of deal for feats.
Admitably He-Man doesn't come off to good to Superman but he seems to have been holding back (for two reasons, going by what happens with zodac and how He-Man wouldn't be trying to kill his ally, though considering he came at Superman with his axe thats kind of in doubt but the first thought is a good one) and Superman was forced to be going serious at him so we know he can at least take a couple blows from Superman and not be knocked out.
Here's a scan from the Zodac/He-Man fight to back it up.
Here He-Man gives Zodac an actual; non holding back hit and Zodac just gets up and in affect says "You think this is over, think again chump, its just begun" and He-Man stands there slack jawed because no one has ever just shrugged off a blow from him before and while this was a serious blow it wasn't all out hard as he could get.
This is his second appearance; the issue after his fight with Superman by the same writer and less than a month later.
Here he opens a hole to another dimension in an attempt to get into Castle Grayskull; it would have worked if Trap Jaw hadn't fallen into the one time hole and ruined it for Skeletor in the other dimension.
Here he uses the gem of the caverns to focus his pyschic energy so he can lift the Talon Fighter from the castle; which attempts to keep it by grabbing it with energy hands and he manages to take it from the castle.
Not really a great pyschic feat but an indication he has TK powers, which he has shown already with Merman.
Here he manages to out Plot Damon; a sorcer who had thought he was manipulating Skeletor into getting the talismans so he could use them to enter Eternia but it was Skeletor who manipulates him, not letting him know he knew of his plot the whole time, intially pretends to let Damon beat him, manipulating him so they could fight near the castle and the release of energy would allow him to enter it.
Here Skeletor talks about his past, also backed up by Zodac thinking about Skeletor at another moment; if you can't read it here's what it
says, also in preview page 13 he erects a force field:
Skeletor: I have traveled the length of the multiverse wrecking havoc...bringing terror where ever the shadow of Skeletor's boot fell across the cowering populace. Billions of souls...Countless worlds, universes beyond counting, all have fallen to my might!
"By all that is unholy how can this be! How can simple wall of stone and masonary defy one who has conquered universes!"
Zodac:...Risking the wrath of forces even he does not quite comphrahend in the name of conquest of yet another world!
Albeit not quite as grand as Skeletor's but it does imply that he has conquered multiple worlds.