Rogue PC, elemental boss fight

[Agahnim]

Hi GitP,
I'm planning to pit my players against an Immoth. I really like their flavour, it inspired me to rewrite part of my main plot, but they're OP as hell for their CR. There are only 3 PCs, they're only level 7 to 8, and they're all unoptimized martials, but the Immoth has been nerfed and is open to diplomacy. I can reasonably hope for this not to turn into a slaughter.
I should add that I haven't nerfed the Immoth too much because, unoptimized as they are, my players always seem to plow through the encounters I give them, and every time I tell myself that I should send stronger monsters their way. So this is it. Besides since it's a boss fight, I won't feel bad if someone dies.
My only problem is that one of the PCs is a very basic TWF rogue whose player doesn't even suspect that some enemies are immune to sneak attack...
How can I make him have fun during the climatic encounter ? I can't have him conveniently find the right item : they're at the door of the Immoth's castle, and besides it would be stupid to field an elemental only to remove his immunity.
Let me reword my question : what can a rogue do against an elemental, or how as a DM can I give him something to do without it being too obvious ?

Edit : for reference, the Immoth is a water and air elemental from the MM2. It is supposed to be CR 9, but on top of his nasty paralyzing poison and perfect flight, it casts as a sorc 12 and has free quickened spells prepared as "ice runes".

[Deadline]

There are a couple of alternate class features that let a rogue use his sneak attack against normally immune creatures at half the amount. There is also a weapon crystal that does this for Undead, as well as a couple of spells. I'm not sure if any of that works for Elementals though. If his entire schtick is sneak attack damage, I'm not sure there's much you can do. You might be able to provide so items that do elemental damage that the Immoth is weak to (it's got the Cold subtype, right?).

[Brova]

The best solution would be the Rogue special ability Crippling Strike. It triggers whenever an opponent is damaged by a sneak attack, regardless of whether they take the extra damage, and deals two points of strength damage. A well played Rogue can drop eight or ten points of strength damage in a round, which is a pretty good option even against things normally immune to sneak attack.

Unfortunately, that's a 10th level Rogue ability.

But you can do something similar. Maybe some of the treasure from an earlier encounter is a weapon that has a 1d6 Dex damage rider. Or you could have the PCs encounter the corpse of someone who tried to slay the Immoth, which would explain gear that counters him (maybe a wand of a custom spell like grave strike but for elementals).

You could just announce that Immoth is actually an Outsider (it certainly looks like one, unless I'm remembering wrong) and not immune to sneak attack.

Finally, you could add a couple of random tundra beasts for the Rogue to deal with.

[Deadline]

The best solution would be the Rogue special ability Crippling Strike. It triggers whenever an opponent is damaged by a sneak attack, regardless of whether they take the extra damage, and deals two points of strength damage. A well played Rogue can drop eight or ten points of strength damage in a round, which is a pretty good option even against things normally immune to sneak attack.

The ability specifically says that the opponent must be damaged by her sneak attack. It's iffy that this would work on the Immoth.


Finally, you could add a couple of random tundra beasts for the Rogue to deal with.

Oh yeah, this is a solid suggestion (and so is the slain adventurer's gear). Non-elemental minions that the rogue can annihilate with sneak attacks will keep him feeling useful without sacrificing the Immoth's immunity.

[Brova]

The ability specifically says that the opponent must be damaged by her sneak attack. It's iffy that this would work on the Immoth.

Well, it doesn't stop being a sneak attack just because the creature you target is immune to precision damage. Think about casting a spell on something with spell immunity. You still cast a spell, even if the target was immune.

[Telonius]

Skill challenge! Give him something to do while the others attack. He has to climb to the top of the gears, balance across a wire, and disable the doomsday device. Or destroy the crystal that's powering the elemental's attacks. Or whatever else might be applicable.

[Deadline]

Well, it doesn't stop being a sneak attack just because the creature you target is immune to precision damage. Think about casting a spell on something with spell immunity. You still cast a spell, even if the target was immune.

Mostly my concern here is that it sounds like the wording is vague enough that some DM's may decide not to rule that way. It's certainly possible that the OP would be ok with it (given that they are looking for ways to pump up the rogue).

And because they are fresh in my mind, you could also make the encounter with the Immoth an Encounter Trap (Dungeonscape p. 120). The rogue spends his time desperately attempting to shutdown the traps while the other party members keep the Immoth off of him. Perhaps the Immoth could be a prisoner bound to the trap, which could explain both the presence of the trap and the limited powers of the Immoth. I ran a desert encounter with a mummy that was like that once. The mummy was trapped and bound to a central platform in a tomb, but could do things like releasing scarab swarms, minions, and environmental hazards upon anyone in the tomb. The mummy was trivial to deal with if you had ranged weapons, but you had to wade through everything else first. And when the mummy was finally killed, the eldritch machine he was bound to shut down, and release the Walking Doom of Ka'amun, a gargantuan half earth elemental monstrous centipede. Good times.

[Curmudgeon]

The ability specifically says that the opponent must be damaged by her sneak attack. It's iffy that this would work on the Immoth.
No, it's not iffy at all. It won't work unless the Rogue also has Savvy Rogue feat (Complete Scoundrel), which specifically makes Crippling Strike work against the sneak-immune.