Hi. I've been playing this game for quite a long time now and just wanted to post some more tips.
If you have a good weapon keep fighting Gnoll Brutes in Caves for their gold. You should get enough to buy up the whole D-11 shop.
If you're invisible and trying to go through a piranha room go one step at a time so you don't go rushing at the pedestal and bump into one and end your invisibility.
I know searching takes up loads of turns and I'm still starving because of it, but searching every regular floor tile from D-2 down has probably saved me hundreds of times. Especially in Dwarven City, you won't want to step on a summoning trap. Died on my best run on D-19 when I stepped on one and summoned a golem and a fire elemental. They didn't kill me but I ran away and got killed by a dwarf warlock.
In Dwarven City buy a scroll holder from the D-11 shop before you go to D-16 so your scrolls don't burn from Fire Elementals.
Again in Dwarven City, if you have a spare weapon you might want to leave one near the stairs of a depth that spawns Dwarf Monks (go for D-18) so if you lose your weapon somehow you can try and bait him to the stairs.
Usually I think Ankhs are a complete waste of gold, but if you bought one make sure you leave a weapon on the previous depth if you're about to die because if a monk kills you before you get a chance to pick up your weapon, you'll still have something when you resurrect. You're screwed if you have no weapon at D-19.
For Tengu just chuck bombs and golden bees at him. No probs.
I haven't died to DM-300 before, just turn invisible then find a spot for Earthroot seed and surround yourself with DoT seeds then use a Scroll of Challenge to bring him over. Just use potions of purification so the gas won't damage you. If you have a good weapon you can probably get him without using any Potions of Healing. Just DON'T LET HIM ON THE TRIGGERED TRAPS! He can basically undo half your damage in one or two turns on the traps.