I hate Save-or-Die with a burning passion, but it's clear that I'm in a minority. However, instant death effect can ruin games, and did so for decades when they were par for the course.
This is as attempt (crossposted on google docs) to make a coherent ruleset that allows Save-or-Die effects, but avoids 1) DMs accidentally TPKing cause they don't no better, and 2) easy experience because monsters whose Save-or-Die is neutralized don't have any other threatening abilities.
Some monsters in this bestiary are listed with two different experience values For instance, the Medusa is listed as XP 500/250. This indicates that the monster is a “puzzle monster.” Puzzle monsters generally have attacks, defenses, or features that make them exceptionally dangerous, but that can be bypassed by relatively simple means.
In most cases, PCs should have advance warning of some kind that a truly deadly monster lies in wait. This might not stop them from charging in, regardless. Fighting a puzzle monsters without any preparation or strategy is about as difficult as the higher number suggests. More likely, players facing these warning signs will retreat and regroup. Once the party has learned the monsters secrets and prepared for the fight, the combat they face will on-par with other encounters of the lower experience total listed.
Generally, a party should be rewarded with the higher XP total for defeating their foe even if they “solve” the monster's puzzle and have an easy fight. Players should be rewarded for overcoming a challenge, regardless of how they chose to do it. DMs should, however, use the difference in XP total as a guide to the difficulty of discovering or acquiring the key to a puzzle monster's defeat. The dehydration vulnerability of a 300/200 XP Sea Troll is likely in the city library; the finding the only weapon that can penetrate the armor of a 1000/300 XP undead warrior could be require an entire session of investigation, culminating in a battle against a dark cult.
A puzzle monsters can be used in an adventure as a normal monster of its higher XP total. However, DMs should be very careful when doing so, as certain special abilities can be very difficult to balance. Even a high-level party can roll poorly and be wiped out by a Basilisks death glare; conversely, a Troll could pose no threat to a tough party, but take dozens of rounds to put down through its regeneration shield.
Conversely, if a monster has its special advantages negated without significant effort, it can be included at its lower XP total. For instance, the king could hire the PCs as Rakshasa hunters and preemptively equip all of them with blessed crossbow bolts, or a party of dwarves immune to poison could stumble upon a demon surrounded by a deadly cloud of toxins. Since those PCs didn't have to solve the monsters' puzzles themselves, they earn experience as if they were fighting normal enemies of the lower XP total.