This guide is out of date as of 2023. It carries some of the same principles but the layout and NPC/Monster names have all changed.
Purgatory Leveling Strategies
This mostly assumes that you are duoing. If you are not duoing you can adjust the guide and kill easier things but this is generally how I progress through.
Overall Zone Advice: Loot everything you can that stacks for stashing in your bank once you leave Purgatory.
Level 1: After you start, claim your AA's, pick whatever race and appearance you want, and memorize the level 1 spells from your spellbook, and you are ready to go. At level 1 I typically just fight prowlers, looting dirty leather and primarily looking for sharp fangs to equip into both my weapon slots if possible. If prowlers are too hard at level 1, just fight a few "a tortured asrai" till you are level 2, then you should be ok.
Look for: an icy prowler
Loot: Dirty Flesh, Sharp Fang
Note: You may also want to kill some "a sneaky ratman" and loot Sticks to save for later crafting.
Level 2: Once your duo has sharp fangs (or bare fists if monk), they can probably take on "a wayward familiar". These are excellent to kill as they give you significantly more xp than the prowlers and they also drop coin and nectarnuts. After you kill a couple you should have enough coin (little over 3g each) to get your characters "crafting supplies" from the skeletal merchant "Serptious Vindavious". This will let you hold enough loot and also do some combines. If you look around the starter area there is also a room with a gnome named "Avens Reogia" in a room with a bookshelf. If you target the "a hard to reach book" and hail it, you will get that book. You can read it if you want, but if you trade it to Avens he will give you a nice +1 hp/mana regen necklace called "Friendship Necklace". The book respawns real fast so you can do it with both characters.
Fight: a wayward familiar
Loot: coin, Nectarnut
Gear: Friendship Necklace
Level 3-5: At level 3 you can probably take on the sorcerers that are worth 3-4x the exp of the wayward familiars. I'd honestly kill a mix of the two as the nectarnuts from the familiars are quite handy in making food to boost your HP/Mana Regen (Spicy Nectarnut Seeds or Cool Nectarnut Seeds). The sorcerer's drop the other ingredients needed to make the HP/Mana Regen food as well. You could stick this out to level 6 if you really want to, it will just be quite a grind at that point, but you will have lots of coin and ingredients that you will be glad to have later on. If you are a caster class I would also hunt "a frosted recluse" spiders to get the rare drop "Ring of the Frost Spiders."
Fight: A sentenced sorcerer and a wayward familiar
Loot: coin, Nectarnut, Rimeweed, Emberthistle, Frozen Chromium
Gear: Ring of the Frost Spiders
Level 6 - 10: You should be able to fight "a damned soul" now. These will drop lots of bones and some copper ore (common) or copper ingots (rare). They are worth roughly 2-3x the xp of the sorcerers when you first get to this point.
If you want to spend some time gearing up instead of purely leveling, you can farm up some Dirty Flesh from the level 2 An icy prowler or level 5 a frosted recluse from the start and craft up some leather/silk gear that you then combine with Frozen Chromium to make the Cold Leather or Frozen Silk sets (Tailoring).
Fight: a damned soul, a gem gnasher
Loot: Copper Ore, Copper Ingot, Amethyst
From here you can crawl through Purgatory murdering every damned soul and gem eater (they are a little tougher) you see till you hit level 10.
Level 10: To get out of here you need to talk to the quest guy (Dizian Maltor) and do his quest. He will also give you a Spiritstone which is a nice item that you can keep upgrading as you progress (Spiritstone Guide). For his quest you will need to loot Quasar Energy from the monsters at the end of Purgatory. They look like wisps on a table. You will need 3 per character to get out. You will also need 1 copper ingot per character.
First he will give you a Drained Power Source which you will combine in your crafting supplies container with 3 quasar energy. Then combine the result of that with 1 Copper Ingot. The result of that combine is what Dizian is looking for, turn that in and you will end up in Purgatory.
Note: Make sure you do all the combines on each character before turning in, in case one of your characters has been looting all the items, you don't want them to be the first to turn in and then get their 1 way ticket to Nexus with all of your tradeskill mats leaving the other character behind without quasar energy or copper ingots.