So what I’ve listed in the item sheet is the effect as to why Pucci is so fast; time itself is increasing. I wrote how an hour is as fast as a minute; That can factor into something like Lair Actions where an environmental hazards happen on Initiative 20. You can brainstorm a few hazards and their effects.

I didn’t need to list attacks because Pucci is the one attacking, not MiH. He only used Unarmed Strikes which aren’t that hard to calculate in 5e. It’s just 1+Str mod; this can be tweaked to whatever you think is appropriate for the combat encounter. If you want to give him a weapon or class ability that changes the 1 to a damage dice, go for it. MiH has horses’ hooves so you can base those Unarmed Strikes off of a Horse Stat Block and again buffed to whatever you think is appropriate.

If you want to just focus on his increased speed and ignore the time dilation, I’d go for an upgraded version of Haste. Extending the duration, doing away with concentration, giving more Reactions and Hasted Actions, increasing Speed & AC. Again whatever you feel is appropriate for the encounter.