>>42619702In Stadium the unevolved rental mons would have more powerful moves than the fully evolved ones. If you've got a pokemon with low offensive stats, give it attacks with higher Power and give high-offense mons get attacks with less Power as a trade-off. High-defense mons get single-stage buffing moves while low-defense mons get sharp/drastic buffs.
For example: Give Ledian T-Bolt, Bubblebeam, Ice Beam, etc, while Vivillon gets Confusion and Silverwind. Base movesets around the stats of the final evolution instead of the current stage.