Meganium’s Leaf Trans lets you move your Grass and Rainbow Energy freely, allowing you to preserve all of it when healing your Pokémon with Blissey’s Blissful Nurse. Since you’re free to move your Energy as you please, you might as well take advantage of Roserade’s Energy Signal, leaving your opponent’s Active Pokémon confused—sometimes poisoned, too—before moving the Energy to your desired attacker.
Tip: Use Seeker to return Rainbow Energy to your hand, allowing you to re-use it for Roserade’s Energy Signal.