The +30% damage on your first strike could become tricky to calculate because of Attack Powerups and Human Glyph Conversions. Hover your mouse cursor on the Attack icon to display Base Damage and Damage Bonus. It's essential to have the correct calculation when you plan on beating a high level monster.
When you are about to drink a potion, be sure that you have exhausted both your Health and Mana so that you don't waste the potion's effects. For this reason, thieves work best as hybrid characters.
Since thieves have a larger pool of items on the map at first, and both potion types heal magic and health, they can usually get more out of exploration even when at high health/magic, so don't be afraid to look for more items.
Race and Deity Choices
Since potions have exactly the same effects for a Thief, being a Halfling and being a Gnome are essentially the same thing. Be one of them. The other races simply can't compete with the utility of that many health+mana potions.
In order to make the best use of the Survivor trait, try to avoid gods that emphasize one kind of playstyle more heavily over the others, like Taurog or Mystera Annur. As long as you keep that in mind, you're pretty much free to take whatever deity you prefer.
With that said, there are some situations where going all-melee or all-magic is beneficial. One example would be if you find IMAWAL, Mystera Annur, and the boss early on. Since the Thief gets an extra Mana-up, as well as two extra potions and health potions functioning as mana potions, the Thief can cast a lot of spells. Additionally, the Stabber trait can oddly override the one damage penalty, so you can still melee weak enemies.
Due to the appearance of extra gold, the Thief is suitable for "Gold Farming": playing a "normal" dungeon simply for the purpose of collecting gold for use in another character's dungeon crawl. Simply romp through the dungeon, exploring as much as possible, while taking out monsters of equal or lower level and picking up every pile of gold you can see. Do not spend any gold on items, of course. If you haven't filled up the pool and cannot explore any more, simply retire and repeat with another thief.
Since this level has both Serpents and Nagas who are anti-melee, you'll naturally want to find BURNDAYRAZ as soon as possible. If you can find the altar of Mysteria Annur the entire dungeon can become somewhat easier. However, what you really want is the BLUDTUPOWA glyph and a low-cost spell. Get Mysteria's Magic boon, activate BLUDTUPOWA, and start spamming that spell as long as you can safely explore the dungeon for more mana. Get the Faith boon as soon as it becomes available, and continue until you can get Flames. From that point on the rest of the dungeon is a joke.
Having the Glowing Guardian early on will make this a breeze using either a Halfling or a Gnome thief. Simply pile up your potions until you can face the bosses while getting a huge power boost from Glowing Guardian. The bosses will easily go down with fireballs and melee attacks. Just make sure that you first bring down BOTH, your health and your mana, before you use a potion and you should be able to clear out the whole crypt.
Without Glowing Guardian things can get a bit nastier. Jehora Jeheyu and The Pactmaker are always helpful, Taurog will power you up but will leave you without helpful fireballs. Tikki Tooki nets you an extra potion and Dodge Bonus and of course you can try an magic only run with Mysteria Annur.
Having no Glowing Guardian it will be a big help to make your attacks magical to beat the Tormented One.
Of all the special dungeons, this should be the most straightforward for you. The general strategy for normal dungeons works just fine here; no extreme alterations are required for a successful run. The only thing to watch for is the crazy-high damage inflicted by Warlocks and especially Aequitas. If you're at a lower level than they are, they may very well be able to drop you in one hit. If you intend to hunt higher-level Warlocks, you'll need a way to reduce or avoid that magical damage, the CYDSTEPP glyph, and/or a high enough max HP to survive at least one full blow from them. For that reason alone, you may find it worthwhile to try playing a Dwarf if you're having trouble clearing the Library with the usual Gnome or Halfling Thief.
The Factory is relatively easy with the Thief. Pile up Potions (as Halfling or Gnome) and slash your way to the bosses. A combination of HALPMEH and fireballs provides enough endurance to kill high level enemies, just make sure you don't use up too many potions. If you have APHEELSIK use it to take on Super Meat Man at around level 6-8, if you have enough free tiles to regenerate. When it's only you and Iron Man, you can convert your fireballs, for an extra potion and afterwards simply outlast him.
It's not really important which deity you have. Tikki Tooki gives an extra potion and a few free hits. Jehora, Binlor and the Pactmaker buff your stats and Dracul gives Life Steal (an option but not that helpful).