Challenging Enough?

[Raethlion]

Greetings all,
I'm in the midst of creating a campaign for my players and was wondering if the reoccurring villain was a high enough level. The group consists of 4 to 6(almost always 4) players; the PCs are all level 8 now. Is a level 11 Rogue enough of a challenge for the group by himself or should I either a)make him a higher level or b)throw in some minions during the encounters? I want him to be beatable but only if the PCs do a really good job.
The villian is a rogue that is a sap master and focuses on nonlethal damage. The PCs consist of:
Human Wizard (Illusionist)
Half-elf Rogue (TWF)
Human Wizard (Blaster)-Not always present
Human Artificer
Elf Ranger (Archer) -Not always present
and an unknown PC (most likely a bard)

PS-I'm sorry for any spelling or grammatical errors as I am typing this on a phone.

[TOZ]

In a straight up fight, action economy will cripple him. While the blaster may have problems affecting him, the illusionist can probably neutralize him, and he can't sneak attack the rogue at all.

Making him a puppet master who works with minions to avoid getting into straight up confrontations will help. If he does have to fight, he'll need the support of said minions to help achieve sneak attacks and avoid being the focus of the party's full offense.

[BEGS]

i doubt a solo rouge would ever challenge any party.
If you want them to feel it use a invisible stalker as a flanking partner for the rouge.

[Mike Schneider]

Such an adversary would probably attempt to kidnap PCs one at a time.

[Secane]

I suggest you incorporate story/environment elements into the fight.

Maybe they are fighting the rogue in a maze like dungeon with mirrors/illusions all over the place. The first time they see the "rogue" and attack, the "rogue" might just be a statue or a illusion.
The ideal is that you still have just 1 boss, but due to the environment the party have to go the extra mile to defeat the boss.

Lava, rolling boulders, traps, falling ceilings, a puppet tent in a circus, animated plants(environment not monsters), wild magic, collapsing demi-plane... etc ...etc. The list of options other then just throwing minions at the party is endless.
AND you can incorporate these into the story. Escaping from the circus while the rogue darts in and out of the crowd to strike at your party members is an example. Do they try to find him? Who can they ask? Where is the exit? Such questions/problems faced by the party can spur on RP and make your story more immerse.

[Ma Gi]

Yeah unless backed into a dead end with no other options, a real rogue wouldn't fight. Remember they are the kings of dirty fighting. So use every dirty trick possible. Traps, sneak attacks, minions, illusions, poisons, etc.

[ZeboJQ]

I would have several minions, along with the rogue. But, before the fight one of the minions acts like the BBEG, while mister rogue quietly stand behind him in nondescript leather armor. That way the rogue can get of some SA before the party even knows who the real threat is. And then of course he'll run the second he's been discovered.

[dragonfire8974]

give your rogue huge initiative bonuses and ant haul and a bag of holding, have him pay a kid to lure the pc into an alley alone and after a surprise round and winning initiative he has a hostage

make sure you'd have the PC's permission before you kidnap them unless you think you can get most of your group unsuspecting and alone quickly

you can have your rogue pay someone to pickpocket a pc and run and pay another group of thugs to ambush the pcs when they come after the theif. only 1 or 2 should run past the nothing special thugs or maybe only one will stay behind and have your rogue smash them and you have 2 hostages

you can 'kidnap' some street urchin, and a goody goody pc may come to the rescue and you can get number 3 maybe depending on if you only have 1 good guy paradigm left

the last idea i have is having some female NPC come on to a PC and as she lures him away you have another

while an 11th level rogue should have a ring of invis and may need a ring of mind shielding too to keep him from being detected when he didn't wanna be. he should only come at them if he has a good chance to knock them out in a full attack plus the surprise round

[Raethlion]

Thanks all for the suggestions. I especially like the illusion/mirror idea. Hope my players are ready for a challenge!