Never heard the spider one.. but the too fast to react death from the elements is well known.
Ok, not sure what exactly is going on there, it might be related to damage types. Did the same fight against the Lighthouse Horror again, this time without losing anyone.
The Horror went against the spider, first did his poison-cloud, but I guess the Spider is immune to poison, so Spidy wasn't very impressed. Then he attacked the spider with his melee attack, but didn't do any damage. This went on for a few rounds until the summon was over. Damage taken: 0.
When he then attacked Roderick he nearly killed him with his poison attack.
Is the LH only doing poison damage? However, the first time it happened with the undead dogs/wolves there as well.