you can't. leaf green is first gen. sunstone wasn't around until second gen. so you can onloy get fire,thunder,water, & Moon stones in leaf green.
You have to use a Moon Stone to make it evolve into Delcatty, it won't evolve on it's own.
Use the pokeflute.
The Pokémon that will evolve once you use a Leaf Stone on it would include Nuzleaf into Shiftry, Exeggcute into Exeggutor and Gloom into Vileplume.
Use a gameshark
If there in the way of your path then use the pokeflute.
use a code
Ditto.
use moon stone
Yes you can.
use thunderstone
Use Cut on the door.
Yes, or a wireless transmitter.