So you built up your last few sessions for that epic encounter with the generic-jerk-boss-that-kills-puppies. Both PCs and the boss are fully prepared for the inevitable combat, with the latter having a good combination of minions to help him. He's got high-tier spells, multi-attacks, cool abilities and sunglasses. Initiative starts and then...
...every PC gangs up on the boss in order to kill him first.
Of course it makes sense. The enemy is clearly the biggest threat and they want to get rid of it first, even if he has Legendary Resistances, 25 AC, magic resistance and whatnot. And all that will result in the boss dying after 2 rounds, and the remainder of the encounter being much less exhilarating.
So what can a DM do, both for keeping the combat interesting and for showing that the big bad was prepared for the PCs, to prevent every single action from targeting him personally?
Please answer from experience and give examples of what you've done and how it worked out - random untested ideas are not useful.