Ah-ha! As being the only person I know who has beaten the purity forest I'll give my advice for the hell-hole....
Strategy for Purity forest
POKEMON YOU USE: Charizard, blaziken, gastly, mew
Charizard: He may lv up slowly but he has a nice move pool and if you find a
couple flame throwers, ect you could go pretty far.
Blaziken: I personally have never used him but he should have a strong movepool and can lv up moderately quickly
Gastly: He may have low hp, BUT he lvs up wickedly fast and if you're desperate can flee through the walls.
Mew: He has high stats, lvs up quickly, and can learn every TM you find making him a keen choice for this go.
ITEMS YOU HOPE TO FIND
pass scarf: passes off all one-square range attacks to a foe
mobile scarf: lets you run through walls (gastly can already do this)
Insomia lens:there are a lot of parasect near the end
big apples: You need to eat
TMs like frustration(45 set damage), flamethrower/bullet seed(long range attacks), attract(leaves foe helpless)
joy seeds: gain lvs
doom seeds: throw at strong enemies to weaken them
gravel rocks: strong throw item for lv 1 dungeons
money: there are many kecleon stalls here!
AND FINALLY
Luck and hope someone rescues you when you need it!