By the time you reach the halls, you should have Magic Map identified and have at least 4 scrolls for each level. Once you enter each one, pop a scroll and beeline for the demon spawner. Taking them out will make the fight against the final boss easier and clearing the levels before that less annoying.

Consider using a potion of mind vision followed by a scroll of lullaby after you've used magic map. Putting a level to sleep makes clearing it a breeze. You may want to use mind vision and a scroll of retribution for really annoying situations you don't want to deal with like a bunch of scorpions. Retribution is more effective if you're low(preferably less than a third) on health.

Light isn't too important on level 21 but you'll generally want to have a torch up at most times to avoid getting surprise attacked.

Having enough armor makes the game much more manageable. It's a good idea to have +7 or +8 scale or plate armor before fighting DM-300. You can beat the game with +1 plate armor but it's not recommended for newer players.

Be wary when running around with less than half health in the halls. Mobs there can easily chunk your health so you may want to top up with healing earlier than you would at higher levels.