the 'highest' composite would be the impossible to get infinity, but if you're thinking about the line that brings up a composite with the properties of every prime (iirc thats in one of the early-mid mining battles) look in the lab, especially the third room. it hints toward that stone. i have gotten it ,if u dont care abt spoilers, i can nudge you further in the right direction.
oh, I see, yeah I've already done that. your post was making me think I'd need to make the number that literally multiplies every prime in the game together, but that would be around 2.3e36. Even with infinite turns and an enemy that always swaps energy that'd be practically impossible for many reasons.