Your best three classes (IMO) for beating Naga City are the Thief (prepare dragon shield), Warlord (prepare avatar's symbol), and Rogue (prepare either trisword or avatar's symbol). I'm sure Priest, Monk and Paladin could also serve you well, but those weren't my classes of choice.
For CYDSTEPP'ers, it's Jehora Jeheyu gnomes all the way. You can convert out once you have 25 max mana, but that number is important since it means one mana potion gives you 10 mana. One potion, one CYDSTEPP, so it's nice. The thief will be pulling the HALPMEH routine instead, so top your resistances by any means necessary. Play Halfling and scum for the Trisword (since you have to prepare dragonshield). You have more flexibility in deity choice than the CYDSTEPP'er here, and on the top-level really anything will do so long as you descend with a couple good boons already purchased and a lot of piety.
what boss to use Whoopaz on...
Okay, you WHUPAZ either Frank, Bleaty or Wall of Goo. End of story. If you're a CYDSTEPP'er then Bleaty isn't a big deal for you. It also depends on which bosses spawn with these guys. If you get multiple bosses that are going to take a lot of resources on the same run, you probably want to bring out your WHUPAZ here.
Yes you need fireball for the end
What do i need to save potios for? Do i need fireball untill the end? What are the major DON'Ts?
. Converting it is a major don't
. The final boss has five layers of death protection and mana burn, so this is pretty much the only use of mana you got against him unless you're mana burn immune.
At what level to descend?
Oh, that's an easy one. Kill everything you can on the top-level without incurring any weakening. It should be entirely possible to completely clear the top level. Maybe a high-level Naga might remain, but don't take a hit of weakening to finish things off.
The biggest trick to Naga City is conversion antics. The bottom level has every single alter at your disposal, so you need to convert and maximize your boon usage. Your top-deities are Jehora, Glowing Guardian, Mystera, and Dracul. The other alters are targets for desecration. Use Mystera as your "potion drinking" deity, and you should be able to build up 100 piety with fairly easily as you cast spells and expend your potions (remember to leave a couple mana potions for the last boss). Mystera is great because those first few hits of "refreshment" are cheap and a great way to conserve your limited tiles down here. You want to swap over to glowing guardian to grab a few levels of cheap protection and humility at some point. Make sure you don't consume potions under him and do any item/glyph conversions at this time. Once you've gotten your value out of protection, move on to your final deity, likely Dracul. You may need to desecrate alters to pull this off. Binlor is easy prey for a CYDSTEPP'er, while Earthmother is easy enough for anyone. Dracul makes a great choice for your final deity, since blood swell and blood tithe can really turn the tides.
Conserve your assets, really make use of that regeneration between rounds when the dungeon resets, and you can pull it off! One thing, though: the final boss gets cowardly at a certain point, and since he both poisons and mana burns you get no regen if he runs into the shadow and you need to explore to chase him. Ensure you do a complete exploration before you begin attacking him.