I have yet again trawled my childhood memories to come up with seeming unusual. I'm not going to tell you just what the crossover is, as I don't want to ruin the surprise...
Act of last resort
The construction site was a mess: Glory's minions and the army of zombie-like humans she'd created had put up tough resistance before being subdued. Thankfully, most of the humans had been taken down peacefully.
All that remained was Glory, currently in the persona of Ben, the unfortunate young man who was cursed with being her living prison.
Giles walked over to the young man, knowing full well what had to be done, and that Buffy would never be able to bring herself to do it. Part of his duty as her Watcher was to protect her from those things she shouldn't have to worry about. It wasn't something he was proud of, but it wouldn't be the first time he'd killed to protect someone he cared about.
He saw Buffy, Dawn, Willow and Xander almost as his own children, and he would die before letting anyone hurt them.
Ben moved; sitting up from amid the rubble he'd landed in when Buffy had knocked him across the construction site. He looked pained, and there was an unpleasant moment as his features morphed into those of Glory. The enraged hell-god stood, saw Giles, and back-handed him into a nearby wall.
"You will be the first to die!" Glory advanced on the Watcher, "Any last words?"
"Can I have a banana?" Giles asked.
"Excuse me?" Glory stopped dead in her tracks.
"Can I have a banana? I really like them, and would like just one more before I die."
"What ever..." Glory shook her head as Giles pulled a banana from his pocket and pealed the top off. Closing his eyes, the Watcher bit into the fruit and swallowed the end whole.
A blinding flash of lightning filled the construction site, and an almost deafening roll of thunder echoed across the clear Californian sky. The entire universe felt like it had been momentarily stretched to breaking point, then let go like a gigantic elastic band.
Glory recovered her senses and looked round: Giles was gone, and in his place stood a tall, muscular man dressed in a blue all in one suit with a hood that reached down to cover the upper half of his face. He wore yellow gloves and boots, and a split yellow cape hung down from his shoulders. There was an unmistakable banana-like quality to the outfit, emphasized by the large red B that served as a belt buckle.
"And just who the hell are you?" Glory asked, "What did you do with the annoying human?"
"I'm Bananaman!" The stranger puffed out his chest as far as he could.
Glory looked at him, and then punched him as hard as she could. Her fist connected with a chin that looked like you could use it as a set-square and her eyes crossed as she failed to move him at all.
"I wouldn't try that again if I was you." A voice called out from above, and Glory looked round to see a large crow looking down at her, "Big-B there has the strength of five hell gods."
"Yes." Bananaman nodded, "Five BIG hell gods!"
A crashing blow from a huge right hand lifted Glory up off the ground and sent her flying through the air. The dragon that had appeared out of the open portal swooped down and plucked her from the air, swallowing her in one go.
"Well, that's not something you see every day." Crow fluttered down and landed on Bananaman's shoulder, "Haven't seen you in a while either."
"Yes; I've been away." The superhero smiled, and then he looked up, "A damsel in distress!" He swam through the air, reaching Buffy just as she passed out of the portal, catching her in his muscular hands.
"What the..." Buffy blinked, surprised to be alive, "Who are you?"
"I am Bananaman: ever alert for the call to action!" The stranger drifted down to the ground, "Now, if you'll excuse me, I must be..."
There was a moment that felt like the universe had been scrunched up like a piece of old paper, then pulled back into shape. Buffy grabbed her head and waited for the world to stop spinning round: Bananaman was gone, and Giles lay spread-eagled on the ground, a dazed expression on his face.
"Ok." The Slayer looked at her Watcher, confused, "What the hell just happened? Where'd Big Blue go to?"
"Now that could take some explaining." A large black bird landed on a nearby trestle and looked at Buffy, "Hi, you can call me Crow..."
As a certain cartoon pig used to say: that's all folks!