The easiest way to nerf Jarvan without destroying him would be to remove his invisible power/utility

Remove the 10/14/18/22/26% 3 second Armor Shred on his Q Remove the passive Armor and Attack Speed on his E Remove the Attack Speed Aura on his E

There, Jarvan is now still Jarvan but without any of the random factors that make him strong for reasons other than being Jarvan.

He no longer randomly amps his already high damage on his passive and R with his Q while also amping an ADCs damage and sometimes Gnar's damage for 3 seconds

He no longer aids in sieges just by existing