>>53231197If you're playing only with rentals, you don't get Leftovers. The only held items available are the single status berries and the 10 HP Berry.
The best strategies with just rentals in Stadium 2 are
>Selfdestruct/Explosion>Counter/Mirror Coat>Destiny Bond>fast Pokemon with strong movesVoltorb's moveset fits 3 of those 4 categories and is basically the most versatile rental in the game.