I don't like Purity Forest, but one thing I can say for all and any LVL. 1 Dungeons, is bring Gravelrocks. Lots, and lots, and lots of Gravelrocks. Try Zigzagoon, they level up reeeealy fast. And also, avoid Pokemon. Try bringing a levitating/flying/water type to go on water. It should keep a few Pokemon away.
Finally, listen to Marcopolo. Stealing from Kecleon shops in Lvl. 1 Dungeons is not only incredibly stupid, it is also a very bad idea.