Are there any official (as in published by WoTC) feats that allow dealing critical hits and sneak attacks to elementals?
I know of Swift Hunter for Scout/Ranger (if elemental is a favored enemy), but I can't find anything for all classes and targeting elementals.
Some context: my PCs (LvL 20) have been fighting a lot of elementals as part of their final quest. It's part of the long story arc, they've known it was coming for quite some time. Meanwhile, their first choice tactics for dealing damage is a critical-hitting fighter and a dual-wielding sneak-attacking rogue. The duo is supported by a wizard, a bard and a cleric providing clever buffing and battlefield control.
Challenging this tactics did influence my choice of encounters for our endgame. However, my goal was to make the players think outside the box, with no intent of a TPK or anything nasty. The long story arc is of a "saving the world" kind and we want the PCs to succeed with style.
The characters are not "optimized" (we play SRD with very few feats and spells from other sources), they've just reached Lvl 21. The fighter did pretty well against the elementals (combination of low AC of the elementals with high Power Attack offset the loss of critical hits), but the rogue was somewhat unemployed (DR/- being his other issue). He has a custom item allowing to sneak attack elementals, but it's use is still too limited.
For their final level up I wanted to provide the simplest choice – a feat. I have a ready homebrew solution, but I was quite curious if there are other options. They won't be able to learn new spells or buy (or craft) any items before the final boss fight. Rebuilding their characters with ACF or retroactively replacing levels is not an option either.
I was looking for something that would restore – to some extent – their favorite offensive options. In the end, I'd like all the players to have fun and have the feeling they contributed to saving the world.
Wizards of the Coast: No.
Mongoose Publishing: Yes.
Some Friendly Advice
If you can't sneak attack, then you have one thing that you can always do and should be doing, especially at that high of level:
Use Magic Device
Some scrolls of Polar Ray would seriously put the hurt on fire elementals. Some scrolls of Horrid Wilting would seriously put the hurt on water elementals. The cleric, should also consider Imbue with Spell Ability to also help out those with subpar class features - and also help him out as well. The wizard, also utilizing Haste would give everyone an extra attack action and increase movement rate.
Don't think that there isn't any time to prepare. Your wizard can cast Greater Teleport (if he can't shame on him), so that you can at least go to town and buy some scrolls. At 20th level, the wizard and the cleric is simply going to outshine everyone else, especially core only parties. Let them. You can still do your part by aiding the others, distracting your enemies, giving out Potions of Healing, etc.