Go with what you know. But will the Marilith know? Metagaming's kinda tricky.

But if they always open combat with a dispel, the polymorph might not work. Illusions are always nice, given that demons and devils like to be tricky types. Speaking of that, a set of layered illusions on top of the polymorph might not be a bad idea. Minions are good as well. A well-balanced bunch of minions (like an adventure group) might help to keep it challenging.

Using terrain to her advantage is a good route. Also taking away a few of the party's bonuses. Light/dark situations, for instance. Does the mounted fighter ride his mount into caves? Sometimes taking out the mount is easier than taking out the rider. Turning the cave into a horrible place to be for those with two legs instead of a snaky body might turn the tables as well. Not quite sure how to do that, though.

Removing buffs is a must. Leading with a dispel might be the first thing any good opponent might do, especially if they are magic-sensitive (detect magic, etc.) or used to facing powerful opponents. Something more powerful than dispel magic might be in order, though. Disjunction? Greater Dispel?

Edited at 2008-06-07 12:04 pm (UTC)

I just ran a maralith. OMFG.

Read their spell list. The maralith MUST summon her demon allies. Then, she must cast that spell (I forget the name) to buff her allies. Every melee hit on them now causes strength damage to the attacker. It also gives all demons SR 25.

I'd max the number of demons that she summons. You may need to cheat and bring them in pre-buffed.

A marilith swordmaster. Possibly with Tempest (Or whatever the one is that gives you lots of attacks with all your weapons). Vampiric regeneration on the blades. Remember, if the bad guy's expecting the PCs, he can be just as buffed, if not more.

Well, are you just sending one creature at them? Parties will own one creature battles most of the time. You DO have a party of seven, even though there are two cohorts, that's still almost twice the standard party size. Have more than one creature in an encounter. Single creatures encounters will end quickly, unless you have good tactics.

Have some demon allies of hers already summoned there, waiting or summoning friends of theirs? Have them pre-buff if you can. If the PC's do it, its silly for monsters not to take advantage of downtime. Heck, in the pre-buff phase, say she auto-succeeds summoning her Glabrezu buddy, who summons another one.

Use her spell-like abilities. Unholy Aura will help a lot. And it will be made more menacing, with lots of creatures to cast it on. Glabrezu's have Chaos Hammer, Unholy Blight and Mirror Image, and Confusion at will. Use them...

Be nasty with these creatures. They are demons for crying out loud. Evil with a capital E. Play that up, and play dirty. PC's don't get XP from summoned allies. So whenever they defeat the Marilith and her hoard of friends, you can tell them they only get X amount of XP, since the others were summoned.