Back of the Turtle: Flashback and Forth.
"Right about now, I can't help but wish for a little help," Flash said as the Justice League stared at the approaching horde of parademons as Darkseid's invasion of Earth began in earnest. The invasion began, not from dozens of boom tubes scattered throughout the world that they could counter and close, but from one huge portal disgorging an army of overwhelming force.
"Granted," said a voice that Superman knew all too well.
"That's right, Supes!" the extra-dimensional being said, snapping his fingers and causing a Slurpee machine and a boulder that had been carved into a comfortable sunning spot to appear with two people on it, both dressed in nothing more than bicycle shorts.
"Mother?" Wonder Woman asked in shock.
"Analogue," Xander said, using super-speed to scan the area and figure out what was going on. "My favorite imp! What prank do you have planned for today?"
"The Flash asked for help with the invasion," Mxyzptlk said. "You were looking bored, so..."
"Actually, I was meditating on the slope of Hippolyta's breasts," Xander said.
"Mother!" Wonder Woman exclaimed.
"This version of you is even more uptight than usual," Hippolyta told her daughter's analogue.
"I can spare a couple of minutes," Xander said. "Might I request a super sharp, unbreakable sword?"
A sword appeared in Xander's hand, a match for the sword in Wonder Woman's scabbard. "I don't see why not," the imp said.
Xander vanished and the parademons fell from the sky in pieces, drenching the foot soldiers in blood and burying them in bodies. The Justice League stared in horror as the invading force died between the ticks of the clock like a bucket of mice dropped in a blender on the frappe setting.
"Done," Xander said reappearing.
"I thought this would be funnier," Mxyzptlk said with a frown.
"I'm the not-completely-sane Xander who believes in overkill," Xander told him. "You were probably thinking of a different Xander."
"Slushee?" Hippolyta offered.
Mxyzptlk accepted and smiled after the first sip. "This is good!"
The boom tube imploded as Darkseid called off the invasion until he could determine what had happened.
"If you want funny, the next time Darkseid tries to invade re-direct the boom tube to the halls of the Einherjar in Asgard," Xander suggested.
Mxyzptlk chuckled. "Upsetting the Asgard is a bit dangerous, even for me."
"They'd love it!" Xander promised him. "Ask in advance and see what they say. Loki would think it a great prank, Thor a glorious battle, and Odin a way to keep everyone on their toes."
"We should get back to our prison," Hippolyta said, "before the gods get upset at you playing hookey."
"Prison?" Batman asked.
"Complicated, and none of your business," Xander told him. "We should get back before Raven gets worried."
"Or a sunburn," Hippolyta agreed with a smile.
"Back you go," Mxyzptlk said, "and thanks for the idea!"
The pair appeared back on the beach with their boulder.
"Cool, got a free sword," Xander said, sticking it into a nearby boulder.
"He kept the Slurpee machine," Hippolyta said with a frown.Typing by: Ordieth!