For general advice, you should explore everything, save your upgrade scrolls for better items (while not being too stingy, e.g. investing in some early armor at the start of chapter 2), use consumables (seriously, you can cheese almost all bosses with alchemy, e.g. using a potion of shrouding fog in the first phase of the tengu boss, bee pots in the second, using haste when dm-300 supercharges, invisibility and AoE potions on the last dworf king phase, paralysis and AoE pots on Yogg's fists), try to fight monsters one by one, identify scrolls by turning them into stones, use deduction to identify potions (there are exactly 2 strength potions every chapter, on different floors; potions of healing are by far the most common and drop from flies; every floor with a puzzle has a solution in it), make aqua potions - they solve twice the puzzles with half the effort, use surprise attacks, exploit monster ai which follows exactly in your steps, use runestones or seeds when in an unfavorable situation, use that examine thing on everything you don't exactly recognize, read the wiki. That's about everything I came up with off the top of my head.

Now, about the evasion ring.

On the wiki you can find out how evasion works: you have base evasion equal to 5 + level, each upgrade to the ring of evasion multiplies that by 1.125, and the chance of an attack hitting you is acc / (2 * eva) if the attacker's accuracy is lower than your evasion, and 1 - eva / (2 * acc) otherwise. Here we find that dworf monk's accuracy is 30.

Your evasion would be (21 + 5) * 1.125^8 = 26 * ~2.6 = 67.6. Applying the formula we get 30 / (2 * 67.6) = ~0.22. Without the ring you would get 1 - 26 / (2 * 30) = ~0.57. Which means with the ring you would evade about 4 in 5 attacks, and without it you would evade about 2 in 5 attacks against a dworf monk at your current level. The ring will reduce your average damage taken per round just about 3 times, which is great, but you have less scrolls to invest in weapons or armor (which can nullify most of the damage completely).

Ok, I see the problem. You were wearing a scale armor instead of a +11 scale armor

Think of evasion as a nice bonus, not something to fully rely on. (And honestly same for armor, though it is more reliable).

For this boss specifically I'd suggest stamina potions, toxic essence to AoE deal with the majority of enemies and shrouding fog (2,2,3 just to be on the safe side). Also get stones of aggression for the final phase, and as general backups you can get stones of flock + shock for wand recharge or + psionic blast for straight enemy clearing.

I could never really make evasion builds work. So I'd say you're not alone. I always try to get at least a level 6 plate going into the DM-300 fight. If I haven't found one by then I usually try to gamble one from the Smith.

I've made it through the final boss and returned to the surface with the amulet as a huntress and mage but never a rogue. That said, yes, it is hard, and yes, it takes a lot of attempts. I would recommend trying to find ways of diverting enemies, tanking a little damage, and making yourself invisible. You can get closer to the boss using blink, haste, and invisibility and you can make the other enemies hit him with stones of aggression. You may also want to consider scrolls of challenge, wand of lightning or corrosion, and using the cloak to go invisible for a turn to let enemies dispirse. That said, good job getting that far, try again with what you learned, you will get there.

You've gotten further than me, I've made it to the city before something got me on my furthest run.