I have a few questions: 1) Which Pokemon is the best to get to purity..

1)I use Mew, try to pick Pass scarf along your way and equip Mew with it
That's how it works
2)After clearing Fallen Valley, and if Blue Sky Meadow Friend Area is possessed, go to talk to Medicham and Ekans in front of Whiscash Pond. Do an optional mission, talk to them again. Do another mission, only Ekans will be left there, talk to Ekans, then look for Medicham's rescue request at Pelipper Contact House notice board
3)Rescue Medicham (do the rescue) and he will give it to you
If you lost it, you can pick it at 50F

1:Charizard because at LV.1 he knows heat wave that is very helpful, due to the fact that purity forest has alot of bug and grass types, and it's really helpful to clear out a room filled with pokemon.

2:First, you need to do a random bulliten board mission that will unlock sky blue plains freind area, then you need to complete both nothern range and pitfall valley, then head for wishcash pond and go over to the members, (members, not leader) of team meanies, they will run of, then complete a dungeon and then head back to wishcash pond and you'll see them again, do another dungeon then go back to wishcash pond to only see Ekans, then do another dungeon and Ekans will be at the bulletin board posting a rescue mission to rescue Medicham, you must accept the job to unlock wish cave.

3:Complete the mission to rescue Medicham, (who is on the tweentieth floor) then he will reward you with the wish stone.