Omni-Man attempts to kill a group of Shonen Protagonists, like he did in the Inviclbe show. To be clear, this is the Show's version of Omni-Man. Everyone is in character, not bloodlusted. Omni-Man's mission to kill them, they don't wanna die.
Fight starts in the same area that Omni-Man fought the Guardians in Episode 1, sized up to acomodate all the combatants to move around without stepping over each other.
If Omni-Man speedblitzes, run the fight a second time with his speedequalized to the fastest person on the opposing side.
If the characters all together would stomp him, then remove the strongest character on the protags side until you get to a scenario where Omni-Man can get at least one kill.
EDIt: It appears that I have massively overestimated Omni-Man's speed. As such I have revised the list of combatants in order to make something that is hopefully less of a stomp
Here are the combatants
1. Son Goku (21st Budokai)
2. Naruto Uzumaki (Pre-Pain Arc)
3. Monkey D. Luffy (Pre-Timeskip)
4. Ichigo Kurosaki (Hueco Mundo arc)
5. Yusuke Urameshi (Pre-Sensui fight)
6. Yuji Itadori (With Sukuna)
7. Izuku Midoriya
8. Ranma Saotome
9. Tanjiro Kamado
10. Natsu Dragneel (Original Series)
11. Koro-Sensei
12. Gon Freecss
13. Inuyasha
14. Jotaro Kujo
15. Kenshiro (Pre-Muso Tensei)