We're playing Return to the Temple of Elemental Evil again for the first time in a couple of weeks and I never recapped the last session.
The big fight against the Earth Temple wasn't a huge challenge but wasn't a cakewalk either. One of the side tunnels before the temple had a group of centipedes that were labeled as the pets of one of the evil clerics. The party ninja tried luring the bugs to fight the temple, but I figured since they were pets of the temple they wouldn't attack, but they didn't add much to the challenge of the fight either. It didn't really become an issue, but here's another weird thing about 3rd Edition. The ninja player's reasoning on luring the bugs out was "they're Vermin, they can't be controlled", and when they didn't attack the bad guys another player said "he must be a Vermin Lord." 3rd edition is incredibly Simulationist. Everything has an internal logic, and while that can be cool at times, at other times it can derail a DM's idea. The NPC cleric didn't have a prestige class to control Vermin (which is a game term for the Type of monster, not a generic word), I just decided he wouldn't be attacked by his own pets.
The only thing that kept the fight interesting was that the players split up to take down different threats instead of piling up on one at a time. One cleric went down right away, but the second troglodyte cleric was incredibly buffed up by magic and was tougher to take down, and the chain fighter was having trouble fighting a Xorn that did lots of damage.
After the head cleric and most of the trog cultists were killed the Xorn disappeared. The group decided to sleep in the temple and I wasted too much time checking if the Xorn could coup de grace a sleeping PC. I need to just start playing looser and faster, and hope the playstyle works. One of the weeks when we couldn't get everyone together we played a Dungeon World one shot using a Dungeon Crawl Classics adventure. I felt like we got a lot more play in that one session, even with creating 3 "0-level" characters per player. I need to crib more playstyle stuff from Dungeon World.
Another thing is I need to modify the adventure a bit to be more challenging. I was loathe to change much because this was a flagship adventure for the release of 3rd Edition, but I'm playing with three optimizers and they're plowing through some of these encounters. I don't think the Earth Temple was designed with an Enlarged chain fighter in mind. More ranged combat and some thought to tactics will help, I think.