A Beguiler is more subtle than just having sheer power and overwhelming foes.
I'd recommend thinking ahead, choose your arenas, you have control over where you'll fight, hit the party with illusions, between a silent/minor/major image, mirror image and displacement... you'll be a nightmare to hit, even better, use disguise self to appear as one of the party members and then your displacement + mirror images + you look like one of their allies meaning potential friendly fire issues if you copy his movements. (perform Act & Bluff & Disguise checks). Not to mention if you do get found out you can buy time with things like 'stay the hand'.
Also to get around anti-trickery have misdirection up... it's awesome.
You hit the right foes with the right battlefield controls. Hit your enemy casters with Fort saves such as
Vertigo Field as most Beguiler spells target Will.
The idea is generally the party doesn't know who to hit, when they decide who to hit it might be an ally by it's likely thin air, this makes the fight last a while allowing you to make use of your Save or Sucks... which again, prolong the fight... once you have them weakened you can maybe apply more underhanded tactics such as poison.
People might say long fights with lots of saves are boring and frustrating, well... yes, if it's every fight... this is the BBEG... he shouldn't be rocket tag. He's not killing the party in a turn and the party isn't killing him in a turn. The long fight gives the party a chance to pull off something awesome and make the fight memorable. It also could serve as the BBEG trying to "buy time" whilst his plans creep toward completion.
I'm not much of an E6 builder but all those spells I mentioned are 3rd level or lower, if you can use shenanigans to get higher level spells then you have more options... one thing I'd advise is getting methods to make the spells last as long as possible, especially those with /round per level casts as the whole shtick is making the fight last.
