Spoiler
Usually if the character has really bad stats I start by creating the monsters in SMC, so that I can possibly pass it later on, then if I happen to have a good bolt spell (not magic missile) I go the Bug temple (I usually leave my spellbooks/scrolls in Terinyo right on the spot where you enter it, and usually no one steals them, this way you can cross the river), and kill a couple of greater claw bugs on the top level, and make a quick escape, this usually gets the char to about lvl 10. If I don't have a good bolt spell I usually try clearing out the SMC and then go the the ID to gain a few levels.

After that I usually go for the SMC, the UD and HMV. when I have found the "secret" passage, I go back to Terinyo, again crossing the river, since I now usually have a waterproof blanket, and go and kill Hotzenplotz. After this I go to the CoC and Dwarftown to get the first quest, and possibly a precrowning, depending on if I've found enough herbs to do it.

Spoiler
Morgia Roots, and Moss or Morrellion.
These are the keys to successful spellcasters.
Morgia roots can be eaten in all forms, even cursed. Moss of morrellion needs to be uncursed, but there are tricks here.
Firstly get a square of 4 with morgia roots at the top. See below

"" - Morgia
"" - Morrellion

Now if you are lucky enough to have another block of 4 on the same level with stomafilla, then you are on to a winner.
Get a breeder, worm or jelly etc into a room, lock the door and let if breed out the level. You are now left in peace.
every time the herbs regenerate, pick one corner of the square clean. Each time move clockwise or anti clockwise round the square so that you get the maximum amount of morgia and morellion for your efforts.
With the moss, you need to avoid the cursed, so try the following. Collect lots of moss until you have three piles of 10 each drop all but 1 from each pile. Put each pile in turn into your tool slot, and then without using, try to remove. You will be unable to remove the cursed. Use the 1 cursed moss, and then pick up all piles. You will now have three piles in your inventory. Two with 10, one with 9. The pile of 9 is cursed. Knowing which pile is cursed makes it easy to avoid, and your Dex will quickly move to 25 or 26 this way.

The idea above of getting SMC with only easy monsters is a good trick I use. I go to SMC, if I'm lucky, I find the down stair, then come back out. Then its infinite dungeon for a few levels. If evil, dark elfs are a favourite of mine, then off to hole in the law to start my thieves guild apprenticship. Then puppy cave to start my herb garden, and if lucky get pre crowned.
Village dungeon doing the druid quest, but not to completion, dont want to create an artifact that will stop my crowning, this is merely to get herbs and altars, the quest can be completed later.
Then when invisibility is available, down SMC and UD, but never linger here unless herbs are on easy offer.

If you have done all this, and no altars or herbs, then you are so unlucky. The big room is a hard place to cultivate herbs.

Bottom line, herbs make the game possible for weak characters. I've never finished the game, but my current char has all the orbs, and has a shot at winning. I've never got passed level 15 without a herb garden, so work on that herb square and go for the win.