Help me design an encounter?

[Falcii]

I am trying to put together a boss fight for an adventure I am working on. In the story, the decrepit old man who helped them earlier is revealed to have actually orchestrated all of the troubles the party has faced this far. Upon discovery, he unveils his true form which is... Yup... Mhmm.

I want him to be a magic user of some sort with a definite evil bent, No undead, and enough hp to last more than a round or two. Given that it's low level I'd prefer if his damage output wasn't crazy, but if he has other ways to mess with enemies that'd be amazing. I am really considering a warlock, but I don't know how to make him beefy enough to pose a real challenge without making his damage output way too high.

The party will be consisting of 5 lvl 3 characters, classes are as of yet unknown.

[Geddy2112]

Instead of beefing up the solo old man, why not give him mooks and make him roughly on par with the PC's? Somebody who has been behind all said problems must have magic that helps them do so, or extraordinary skills to accomplish this.

Perhaps he knows the aprty is onto him, and brought along some goons to stand in the way. Maybe he has the ability to control or influence animals and has swarms of bees, or snakes, or even large animals that can kick and bite.

If they end up confronting the old man, make it in a place he has advantages-from as little as high ground to as much as magical traps and battlefield control spells. This is extra important if he is solo.

[J-H]

Make a 6th-level beguiler with some minions. He does no damage himself, but every round somebody is getting slowed, blinded, taking non-lethal damage, getting dazed, etc.
Don't forget to use the immediate-action spell Stay The Hand once per round to force a "Will save or you can't attack me, I'm just a harmless old man!" if they get close to him.

[Gildedragon]

Old man is a:
1) Doppelganger
2) Succubus
3) Rakshasa
4) Evil Fairy
5) Genie

Minions:
Illusions, shadow conjured beasties of all sorts, maybe a couple dominated peasants.

I'd say 3 Stages to the encounter:
6 1HD Enchanted Villagers (Commoners, Warriors, Experts, maybe 1 adept)
A handful of Enchanted rats and dogs (not cats)

4 10% real summons
4 2 HD Enchanted Villagers or willing cultists

1 Rakshasa casting as a 4th level sorcerer (can't use evocation or conjuration OR necromancy and conjuration... Or all 3) either use a full rakshasa or a monstrous class rakshasa; if using a full rakshasa: rakshasa has spent all their higher level slots on other stuff, like creating the shadow summons, or traps like shadow well or knowing pre-metamagic-ed versions of spells they already know
Rakshasa is a coward and will FLEE (perhaps with a shadow well) if they start losing, or if they drop to less than half health.