I had a couple of off-line discussions about my Boss Attack Difficulty idea.
What it came down to was, basically, it's got no downsides, and that's potentially a bad thing.
As written, that's on purpose. It is a pure Power-Up for boss monsters. It makes them flat-out better, and it's not meant to come with balancing downsides.
It's a genre power switch.
There are two options worth mentioning, though:
Only if the boss leaves you alone. This is the "the boss does boss-like things in the background" switch. You get a penalty to attack as long as the boss isn't doing offensive things to you. Summoning a demon, finishing the Kill-O-Matic's Death Countdown, ranting about his evil plan, etc. - fine. Attacking? BAD goes away.
Why I didn't do this: Because I feel like it just makes a Boss + Minions fight into a Minion fight and a Boss fight. They may as well be in separate rooms or otherwise separated. The boss is hampered by this, changing it from a power-up to a temporary plot shielding. If that is what you want, this is the way to go. But it's not really any different than putting the boss behind a defensive screen that only drops when the minions are killed, or otherwise splitting the fight in two. It's at cross purposes with the original goal of forcing you to deal with the minions while taking shots from the boss if you want to level the playing field with the boss.
Weaken the Boss. This approach uses a basically weaker boss monster, which depends on the penalty you have to attack it to defend itself. Instead of a giant death-dealing dragon, you just get a more normal dragon but with BAD -5 so you're at -5 to hit it or enspell it.
Why I didn't do this: Again, it's meant as an unmitigated power-up. Weakening the boss creature but giving a big penalty to attack it works, but this was meant to effectively boost the utility of minions for already-strong bosses. This works, it's just a little odd to give, say, an otherwise not-boss-level Goblin Chief a BAD of -5. I think it will feel a little more meta-gamey, where the boss suddenly feels weaker. With a strong boss, it feels like you're suddenly able to concentrate more than zero in.
Anyway, those are two ways you can mitigate things, and why I didn't choose to do so.