The Naga is a monster found exclusively in the south. He has the Weakening blow trait, allowing him to permanently reduce your base attack by 1 point per Weakening stack. There are very few ways to remove the weakening effect, and normally you will want to avoid accruing any weakening whatsoever. Use glyphs like BURNDAYRAZ and GETINDARE to avoid ever taking a blow from a Naga, and thus evading his nasty debuff. The Naga is otherwise a relatively weak enemy and can be dispatched quite easily.
If you find yourself weakened, you have few options at your disposal. The cleansing boon of the Glowing Guardian can remove a single layer of weakening, but the high piety cost means that this isn't something you can do frequently. The chaos avatar boon of Jehora Jeheyu will remove all weakening, but this is a one-time effect so it must be timed very carefully. Finally, the Fortitude Tonic will remove all weakening, but the supply of these potions is quite limited.
The Naga boss, Itsssama, is only found in the Ick Swamp and has high physical and magic resistance in addition to his weakening blow. There is little that can be done about this, and your best options are to either save him for last or drink a fortitude tonic once you've beaten him. Jehora Jeheyu can be a very useful deity for defeating him due to chaos avatar. If used before the boss fight it will sharply reduce Itsssama's resistances, and if used afterwards it will clear away the weakening and make fighting the second boss easier.