Help with strategy for boss

[Lord Il Palazzo]

Aira, Uial and Ginorbol, absolutely do not read this.

So, last week in my 3.5 game, my players fought an NPC who was supposed to give them a long, difficult fight. Thanks to some cunning role-playing they got a surprise round and a couple of critical hits later, it was all over before the bosses allies had a chance to kick down the door and start buffing the boss and summoning buddies.

Next session, there's a strong chance that the PCs will end up fighting an NPC several levels above any of them and I specifically want the PCs to lose. The NPC is a Yuan-ti pureblood with 4 levels in Fighter and 2 of Battle Sorcerer. The players have met the character but thanks to a magic disguise and the players believing a mistake made by another NPC, they believe he's a human ranger.

The PCs just hit level 6 and are as follows:
Wizard 6: Focus on buffing and debuffing (with a strong liking for Hideous Laughter and Color Spray) and a little blasting here and there.
Fighter 2/Ranger 4: Two weapon fighter with a focus on mobility (with Spring Attack and the like). Thanks to a collective misreading of the rules, she's been allowed to use each weapon on a standard action attack and this was recently codified as an alternate class feature in exchange for not getting magic (and generally taking a weaker build). Just got her wolf companion.
Druid 4 or 5 with major djinni bloodline 2 or 1: Typically rides a leopard companion into battle and goes melee with buffs from the wizard or with Shillelagh. If he skips his bloodline level to get wild shape sooner, bets are generally off. If he doesn't 'shape, he's small sized (a gnome) so tripping should be easy.
Rogue 4/Swashbuckler 2: A former PC running as an NPC until there's a good story chance for her to leave. Focused for now on ranged weapons with a quiver full of various magic arrows.

There's also a level 5 NPC cleric with the party, though he isn't very combat focused; they're escorting him to (and probably from) a magically charged location where he needs to be to brew a special potion/MacGuffin.

Here's what I'm thinking of strategically:
If he has a chance before the fight, the Yuan-ti will use his Charm Person SLA to try to charm the Fighter/Ranger. This will likely work as she only has a +1 to will saves.

As the fight starts, he will Entangle (via his SLA) as many PCs as he can and probably start taking shots with his composite longbow to try to take down (or at least damage) the wizard from range. When a PC escapes the Entangle spell he will switch to a gisarme (+1 and Merciful; he wants to take them down, but not necessarily kill them) and attempt to trip anyone who comes within AoO reach (with Improved Trip, he has a pretty good chance to succeed). He may specifically go after the Figher/Ranger (is she resists his Charm) to trip her with a non-AoO attack as she can avoid AoOs with Spring Attack. He can also make AoOs (maybe trying to trip) within 5 ft, thanks to spiked armor to threaten the area.

If he has a chance, he may take a turn to cast True Strike so he can power attack for his full base attack bonus (+9) the next turn and be relatively sure to hit (for +18 damage thanks to his two-handed weapon).

Ideally, I just want him to incapacitate the party (hence the merciful weapon) so he can interrogate them about some information they may or may not have. I'm not above having him charm the fighter/ranger and ambush the wizard and druid (and NPC cleric) while they're preparing spells (thus limiting them to what's left from the day before).

Does anyone have any suggestions for strategy on this fight? Like I said, I really want this fight to be hard for the PCs and ideally they should lose (non-lethally). Thanks much (and sorry for the long post.)

[Toliudar]

This is a tall order, as presented. The PC's aren't that much lower than this guy, he has no rocket-tag one round take-downs, and there are a wizard and a druid in the party (always increasing the likelihood of random bits of brilliance).

Is it necessary that they meet in a battle? He's yuan-ti, masters of subtlety and poison. Maybe he tracks them down while they're on their way to him, and ambushes them while they sleep. He sends in a pack of minions/disposable allies first to hold their attention, allowing him to plink away at the mage from a distance.

Any chance you could soften up the group with some ability-draining traps? Perhaps the room where they meet is infused with a poison gas that makes mammals exhausted.

Alternately, minions! Minions with nets and spears that aid another on each other's attacks. Minions that plink away at entangled enemies with shortbows. Minions that get in the way of a leapard's desire to leap at the BBEG's throat and tear it out on round two.

How powerful is Charm Person in your world? Mostly, I see it as predisposing someone to listen and be helpful towards the charmer, but that doesn't extend to helping (or standing by and watching) while they slaughter your companions. In short: even if Charm Person is successful, what will that do?

[Lord Il Palazzo]

I know it's a bit of a tall order, which is why I'm looking for help.

Charm person, of course, isn't going to completely turn the ranger against the party (it isn't Dominate Person, after all). It should, however, make it easier for the yuan-ti to get her out of the way briefly ("The casters are preparing their spells, would you mind doing [something to keep you busy and elsewhere] while I guard them?") while he engages the others.

The biggest problem is that the yuan-ti has plenty of access to the PCs, but not much in the way of resources for the confrontation so a lot of his strategy will have to come down to his planning and timing. At present, he is actually travelling with the PCs (having just met and become aware of them a couple days ago) and they see him (as much as they've let on to me, at least) as a gruff but not hostile (toward them, anyways) ally. (As I said, he's disguised as a human, for now.) He's just realized they might have the information he wants (and maybe a MacGuffin he's been looking for) and is trying to find a way to get it. Since he has a reason to be around them, he has a lot of freedom in timing his attack and could even attack while they sleep (though someone's always on watch).

Unfortunately, this yuan-ti isn't in a very good position to have minions of his own. He may have a couple underlings (probably other yuan-ti, though this isn't set in stone) travelling a ways behind the party (so as not to be detected) but that's all he'll be able to muster on short notice. Their assistance will certainly help, if he can get a message to them in time to coordinate.

Another thought: the PCs are likely to have a fight very soon with a fairly high number (probably about 10) of low level NPCs (minions of the boss they took down too fast) in order to hold the magical potion-making place while the cleric works so the Yuan-ti might time his strike during that if he doesn't want to wait for the next morning when the casters are prepping spells.

He believes he could probably get some of the information he wants from the ranger through the charm, but that's only one of the PCs and he feels the others are likely to have more information. (In actuality, the ranger knows far more about this subject than the others, but he has no way of knowing this.)

[Toliudar]

If what the yuan-ti wants is indeed information, then I'd forestall any kind of armed conflict as long as possible (and it doesn't sound like there's anything prompting him to immediate action). Lots of campfire chats, possibly working in charms (although I hate doing that to players), and waiting until they're weakened or separated to strike. Why fight three when you can fight one?

[Lord Il Palazzo]

The problem here is that the players are going to fight the yuan-ti sooner or later. They don't know it yet, but one of them has a major backstory reason to want him dead and they'll almost certainly realize it as soon as they realize what he really is (and I feel like this will happen sooner than later). The yuan-ti is reasonably certain this is coming and wants to strike first to have as much advantage as he can.

I suppose I could have him split with them and try to tail them for a while to pick a better time and place, but the thing is that the campaign is about to shift its focus from questing across the country for a MacGuffin to save a town from a plague to a more urban plot where the character doesn't fit in as well. The yuan-ti could come back into the story later (especially if he thinks there's something to be gained from following the PCs) but I don't feel like he's likely to get a better chance than when they're away from civilization (nobody to interfere) and probably distracted by or weakened from a big fight with (hopefully) a good number of their spells used up.

[JadePhoenix]

Well, a pureblood yuan-ti has 4HD, so you're looking at 10HD vs 6HD. Be advised - like Toliudar mentioned, the odds are against you.

Luckily, the character has already met them, so you can use this for your advantage. He'll be crazy prepared.

First, you'd be wise to build to your strenghts. Yuan-ti get all Knowledge skills as racial skills, so take Knowledge Devotion. You'll only need Knowledge (local) for these guys, so it's easy to max it. Carry a greater skill shard as well. With maxed Knowledge (local), Int 12 and a greater skill shard, you're looking at a +20 modifier.
Second, you don't want to stand still. Get him as many ways to move around as possible. Spring Attack, Quicksilver Boots, Chronocharm of the Horizon Walker, Anklets of Translocation, I don't care, just make sure you can get him to move. Don't stay still, a full-attack will hurt.
Third, use a Merciful weapon. You want to defeat them, you don't want to kill them, right? A merciful greatsword will be doing 3d6 damage, not counting Knowledge Devotion or Strenght modifier.
Fourth, get good defenses. That means high saves against that wizard, nice AC and getting lots of defensive spells. At Battle Sorcerer 2, you'll probably only have Shield and Nerveskitter. You can, however, load up on scrolls and wands. A wand (on a wandchamber) of Wings of Cover will make him last a lot longer. Get scrolls of 24h buffs, like Primal Instinct or Craft Magic Tattoo. You might want to grab a few maneuvers from ToB or some soulmelds from MoI to be on the safer side. Wind Wall is good - you'll get rid of the rogue, and since it's only an NPC, no one will mind.
Fifth, I'd give him max hit points for his hit dice.
Sixth, I'd fight on an environment he gets an advantage. A dark, closed room is ideal. A place with corners for him to hide, get cover from, stuff like this. Have them waste their actions getting their own light.
Seventh, get him a bow. Yuan-ti have poison. Have him fill them with poisoned arrows from a distance before they have a chance to even try to engage him in melee.
Eight, remember even with all preparations, action economy is against you and action economy is king.

[Slipperychicken]

Wherever the party's going, tell the army to set up for an ambush. Big bad plays nice for now, acting like a good, cute little npc. Have the big bad start holding watch, and send the order to strike. Army could "take him captive" while he pretends to struggle. That way the PCs are ambushed, at night, with very little warning (if any at all), and the army has a "hostage" to play around with. If big bad wants to fight the party himself, have him "captured", then walk back without his disguise, and say they have the fake npc hostage.

EDIT: just read spoiler. If the cleric is the mcguffin, drag him off with the bbeg, possibly by warning him that an attack is imminent, and there's no time to warn the others. Now you have two npc hostages.