[Aerich]

Spoilers ahead
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If your thief has a rather bad chance to find the traps, what you need to do is have a cleric or a druid cast the 2nd lvl spell "Find Traps". That will at least let you know where they are; your thief may even be able to remove some of them. Some of the traps on the top level can't be "disarmed" by stepping on them - they will go off over and over, as long as you keep stepping on them. It's quite time consuming, but you may have to try and memorize where the traps are.

For fighting Y, I like to hit her doorway with multiple Web/Entangle/Stinking Cloud spells. Slow is also an under-used spell, but is great for this fight, as is Confusion. If you get lucky, you can trap her in the doorway behind all her minions. A good archer can shoot her to death with the +2 arrows (use a haste potion for quicker results), and a couple fireballs and Oils of Fiery Burning will take care of her minions.

When you cast Find Traps, you will notice that there is a "channel" free of traps down the centre right before her doorway. You can advance a mage or two and an archer down that channel for the preliminaries. If she gets loose from the Webs etc before going down, retreat your two back to the party. You should have a few summoned monsters hanging out just behind your mage and archer, ready to intercept Y or anything else that starts chasing your vulnerable mage. Your mage should have Mirror Image as its sole defensive spell for this fight, although a Dimension Door could also be helpful. If you have another mage, you should be hitting Y with as many single-target damaging spells you have - Magic Missile is good, Chromatic Orb can stun her if it gets through her magic resistance (worth a shot), and Acid Arrow and Icelance are good as well.

Hope this helps. It's an alternative strategy to charging into her room, which I've never been fond of doing, for some reason. I prefer to be set and waiting for them to come to me; I find it easier to keep weaklings alive that way.