I faced the same issue not so long ago. I have a big party (7 players) and it was time to fight the big evil vampire lady wich was a powerful wizard. The players were level 11 so the lady had the potential to DESTROY the team even if i made her lvl+2 since the guide suggests to give her an extreme DC. In the end i avoided giving her instawin spells like high level paralysis and slow, lowered her spell DC and just let her prebuff a bit to mantain the powerful wizard feel. In the end the result was OK (the team rolled exceptionally poorly and I as a Neutral Good master just played the boss a little sub-optimally to not murder everyone), I only killed a single pc that was knocked unconscious while flying way too high and the fight was allright.
If I had to do it now (and I will again eventually) i think i wont change anything, except maybe putting in less minions (6 was a bit much, the fight took forever).