On a more general issue, I've found that when PCs reach levels 10-13 or so that most melee combats last 6 rounds or less unless there is a lot of mobility involved (or lots of foes).
One of the side effects of this is that you only really need to think about the first 4-5 rounds worth of the BBEG's tactics anyway
One tactic I always try to think about it is this - what is his exit strategy? How would he best escape?
Unfortunately I often pick strategies for my monsters which are too effective (while being within the bounds of what is likely for them - almost all the white dragons met have been unsubtle because they are not so bright. Mind flayers and high level wizards tend to be much more well prepared.
One of the most common strategies for those with magical powers is normally:
a) get yourself safe
b) arrange for patsies to attack the PCs and cause them to waste power/display their capabilities etc
c) wear them down with lots of minions and then get them before they can recuperate
When you get into combat - hit with the biggest stuff first, but try to make sure your defensive precautions have all been taken.
A good tactic for fiends and celestials is initiate an attack and once the party has buffed up teleport away to safety and wait for their buffs to wear off before attacking them again
