(really you just treat them like weaken/corrosion monsters and try to one-shot them or first-strike)
It's difficult to do this with the intent of earning bonus XP, and most of the time it's in hindsight cleaning up your popcorn. Certainly you won't be able to take out a Bandit that's 4 levels higher than you, a luxury any class without resistances who finds the APHEELSIK glyph can muster.
The point is, having resistances that you want to preserve is an opportunity cost. This cost is the same whether you're stacking huge amounts of resistances or very few; either way a single hit of resist-down is devastating. As a result, it's been low-resist characters - not the resist stackers - that have suffered most as a result of the newfound prevelance of monsters with the resist-down property.
I also think that occasionally losing 5% on the resists isn't an absolute write-off, especially for smaller resist amounts.
If you take two hits of 5% resist-down while you have a tower shield, then it literally is a writeoff
. You no longer have any resistances at all, and may as well convert the item.
I quite like the way sword and shield function now - a leg-up for the first one or two levels (with vampires I'll keep the shield pretty often), then convert for inv. space.
While I can understand vampire with the shield (since he's effectively a DR stacker), I don't share your enthusiasm for these items otherwise. The advantage is too short-lived given the long-term edge that a perseverence badge will afford you.
I've never really used the monster whistle
And I completely forgot to mention it. That doesn't exactly speak well to its effect.
Bottom line is that people do not
like fighting equal-level monsters. They give no bonus XP but they're pretty challenging at the same time and will drain your health and resources. Maybe make it usable once per level?
Definitely, Ying + Yang is tricky enough anyway.
The subdungeon itself is another matter; I'm speaking about the reward. The Ying/Yang subdungeon takes a lot of resources and/or planning to pull off. Given that it's a no-explore subdungeon and it can only be completed by very high-level characters, the reward isn't nearly good enough.
Also, to preserve the sword without it encroaching too much, how about making it a soup ladle. It's like a spoon, and it's small space!
Could call it the "short sword", make it a small item, and be done with it.