Captain Amazing is dead. Casanova Frankenstein is about to unleash death and mayhem upon Champion City. The Shoveler has given his sandwich speech. The Rag Tag Bunch Of Misfits clamber aboard their Herkamer Battle Jitney, and break into Castle Frankenstein, where they're immediately surrounded by the Disco Boys shooting at them with guns. So ... why doesn't the Sphinx just cut their guns in half with his mind?!
If I'm not mistaken, in his intro scene, he reveals himself as having been standing right to the side of where the line of Disco mooks were standing. So it might be that his power works by line of sight, meaning he wouldn't have been much help without exposing himself to a lot of gunfire.
More importantly, they sat in a tank(or "Battle Jitney"), with a huge electromagnet designed to pull guns out the hands of enemy gunmen.
The whole joke of the character is that while the Sphinx seems to have what are genuinely awesome psychic powers, the only thing the characters — including himself — are interested in is his trite, fatuous pearls of wisdom.
His powers are terribly mysterious; there was probably a good reason he couldn't or didn't do the gun-slice again.
His powers aren't terribly mysterious. His powers are that he's terribly mysterious.
He's terribly mysterious, so therefore his powers are terribly mysterious by extension. The fact that he even can cut guns in half with his mind, even once, is pretty amazing, since it sounded like something Blue Rajah pulled out of his ass to talk him up when Furious wasn't impressed.
The one thing we know about the Sphinx is that his powers are that he's terribly mysterious. The fact for certain what his powers are means that they are the one part of him that isn't terribly mysterious.
This is part of the joke/point; the one potentially useful power he has, and he ignores it in favour of lame Ice Cream Koans. Although like Invisible Boy can only become invisible when no one's looking at him, it's possible the Sphinx can only do the gun-cutting thing if no one knows he's there; like said about, it would tie in with the idea that he's 'terribly mysterious'.
Tying in with the Fridge Logic on the YMMV page - if the Shoveller's special ability is being able to shovel very well, why didn't he shovel a tunnel into Casanova Frankenstein's estate, rather than run along the lawn? Or would that be a job for The Digger?
He shovels well, but you'd need months of time to dig out a hole like that by hand.
Uh, because "shoveling well" is not a superpower? He's a guy with a shovel, and he hits people with it. That's it.
The power of shoveling is most formidable, but it's useless if the enemy's foundations go down to the bedrock. He's not the Underminer, after all.
Let's put it in superhero terms, since it is a superhero movie. He is the Captain America of shoveling... he shovels at the peak human ability to shovel. But even the peak of human ability to shovel can't tunnel from outside an extensive set of grounds and up into a mansion in a matter of hours. So the Shoveler applies his shoveling to things other than shoveling out a hole. And if "shovel" still sounds like a real word after reading that, I haven't done my job.