Getting through Purity Forest depends entirely on the Pokèmon you select to take. Just try and take any Pokémon. If you die inbetween you have nothing to loose since all money and items are stored away in the Town Center. Take Charizard and try it out like others say. You could take Mew since it levels up fast and can learn any TM that you find on the ground. Or you could try it out with an Legendary since at Level 1, they have good stats. Mewtwo is in my opinion the best since it levels up faster than any other legendary. Just go for it since in this dungeon, if you loose, there is really is no drawbacks. The only thing you will loose are items and money found within this dungeon. Here you can't recruit any of the Pokémons except Celebi. Good Luck.