(Disclaimer: I own nothing from Image or DC Comics, nor do I own any of the artwork I have posted or will post.)
Titans Tower:
Mark and Connor met in mid air and immediately began exchanging blows left and right. Mark quickly figured out that he was faster and had more control with his flight than his opponent. He figured he and Connor were about equal in strength at the moment, although if push came to shove he had an idea he'd lose to the Kryptonian in an outright brawl. Nonetheless, he quickly found himself in such a war as he let go and really laid into his blows. Connor did the same, and the two went from attempting to end the fight quickly to seeing who would crumble first. They both knew each other could give as good as they got, and they threw hands even harder at that, the prospect of meeting a foe on equal footing exciting them..
This was much to Supergirl and Superman's chagrin of course, as they and the team watched the slugfest go down on the training floor. Connor had done much to control his temper in recent weeks, but Kara supposed that he was handed a golden opportunity to go all out against someone who wasn't her or Clark, or a villain who he couldn't lay a finger on. She switched her focus to Mark and noticed he had the beginnings of a smile on his face even as he ate a big right hand from Connor and sent a left hook of his own across the boy's jaw. She wasn't quite sure, but she was starting to get the idea that Mark was using this as a way to fully drain his own anger while showing her cousin (his hero) just what he was capable of. She also knew that footage of this would get back to Batman eventually, and she secretly wondered if Mark was using it as a way to show how dangerous he could be if the Batfamily tried to further tamper with his life. After the story she heard from both her cousin and Batman after Mark's initial disappearance, Kara swore to herself that she would try and be the Good Samaritan and be a better hero if she ever met him. Mark was a good kid, Barbara and Tim liked him, and Clark always preached about doing the right thing.
But she'd heard most of the conversation between Clark and Mark. She also heard what he said to Donna after he thoroughly embarassed her. And while not a whole lot of it made sense to her, there was one thing she knew very clearly:
Mark was not someone the League should underestimate or take lightly. Because if they did? He'd make them pay for it. Violently, if necessary.
Turning back to the fight, Mark and Connor were still swinging away. Mark's technique, tactical ability, and understanding about his powers related to his fight style were pushing Connor's durability, adaptability, and strength to the test. The two had separated for a brief moment before Mark half jumped and half flew back into action. He twisted in mid-air, using the momentum to drive his knee into Connor's ribs. The blow landed, but Superboy barely flinched, countering with a quick elbow to Mark's jaw. Crack. The sound echoed, but Invincible gritted his teeth, swinging around with a hook that caught Connor off-guard and sent him careening into the nearby wall.
Mark barely had time to react before Connor was up and back in his face, delivering a barrage of punches. Invincible blocked most of them but felt the raw power behind each hit. One punch slipped through his defense, connecting with his gut and sending him flying. He took off like a missile, closing the distance between them in a split second, and tackled Connor into the ground. The two heroes tumbled forward, fists flying at breakneck speeds, each strike shaking the air around them. They were both holding back, but not by much.
Connor locked Mark in a headlock mid-flight, squeezing with his enhanced strength. "Tap out, Grayson! Before you regret it!"
Mark's vision blurred, but he wasn't done. He reached up and grabbed Connor's arm, swinging him around before slamming him into the ground. Connor got up, twisting unpredictably, and landed a vicious punch to Mark's face. Both spat blood out of their mouths and shot forward again, neither willing to stop. As they traded rapid blows in the air and on the ground, each strike was louder and more powerful than the last. Neither was giving an inch now. Mark managed to land a murderous knee into Connor's chin, and the tide seemed to turn further in his favor until a blur slammed into the ground stopping the fight.

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To Be Invincible (Invincible x DC Crossover)
ActionMark Grayson is a young man who lives with his alcoholic mother in the Gotham projects. When he awakes on his 18th birthday, he suddenly finds that he has new and mysterious powers that make him do incredible things. Which begs the question: who i...