One concept I love in GURPS Action (specifically, in Action 2 - reviewed here) is "Basic Abstract Difficulty," or BAD. Doug mentioned Action yesterday, and this idea sprang to mind.
BAD is the penalty you have to most of your rolls during an adventure. You can whittle this number down by your own actions, especially ones like planning, hacking, finding clues, dealing with side issues, etc. It goes up as you get closer to the boss, as well. (Alternately, it can just be the bonus to the skills of the bad guys, which is important here.)
So the PCs can just drive straight to the boss bad guy's HQ and shoot him, but if the BAD is -8 or so, it's going to be a rough fight even getting past his mook guards. Spend some time cutting it down to -2 or -3 doing the kind of stuff you do in the first 75 minutes of a 90-minute caper or action movie, and now it's a doable task. It's beautiful because it is player-driven - the players want to do all the action movie stuff because it matters, but they can skip it if they're willing to suck up penalties.
You can use this in DF to deal with the Boss Monster and Minions situation. Generally, PCs go for an alpha strike on the boss and then mop up the minions. As anyone who has ever seen any movie or played a side-scroller video game knows, the boss fight isn't resolved with an alpha strike.
Let's find a way to encourage that without requiring specific sequencing (aka, a puzzle monster.)
Basically, we're going to merge (and invert) the concept of the Last Ninja rule (from Fourth Edition Festival) with BAD. Instead of a bonus as the numbers go down, or a basic penalty for non-combat situations, the boss is harder to deal with offensively until you've knocked off their lesser allies.
Boss Attack Difficulty
For Boss-and-Minion situations, set a BAD for the boss. This is the penalty for:
- attacks aimed at the boss (or including the boss, if area effect)
- spells cast at the boss (or including the boss, if area effect)
- attempts to Fast-Talk, stun with Rapier Wit, etc. when aimed at the boss
- general attempts to harm, restrain, or otherwise deal with the boss offensively. Not sure if it counts? Then it counts.
In cases where a penalty doesn't make sense (such as a Quick Contest, or resistance rolls against poison, Mass Sleep spells that cover the boss's hex, etc.), give the boss a bonus equal to the BAD instead.
Noteably, this does not affect the players, unlike standard BAD. It specifically doesn't reduce their resistance rolls or defenses - having someone hold off the boss defensively while you take out the minions is a great tactic.
Divide the number of minions by (-1 x BAD) to get X. For every X minions taken out, BAD goes down. Always round in favor of the boss monster!
Example 1: Netherwail the Dark (cleric-necromancer-demigod) has his eight high priests with him. Netherwail's BAD is a staggering -8; each priest is worth 1 BAD. PCs are at -8 for any attacks against Netherwail, -8 to cast spells on him, etc., but not to any rolls to defend against him. As each of his priests gets killed, the BAD is reduced by -1. If all eight are killed, the PCs can attack Netherwail freely!
Example 2: The Goblin King has seventy (70!) goblin guards in his throne room. His BAD is only -5, because he's not as big of a baddie as Netherwail. His seventy goblins are each worth -5 / 70 = 1/14th of 1 BAD. For every 14 goblins taken out (slain, knocked out, put to sleep, routed, etc.) the Goblin King's BAD is reduced by 1. If the PCs manage to take out 30 of his goblins getting to GK, they would be at -3 to attack him. Eight more goblins would take him down to -2, making him more and more vulnerable as his bodyguards go down.
This can work for non-minion situations - a boss dragon who has its essence in each of four pools (one for each element) that must be drained might be BAD -10 but lost -2 bad for one pool, -5 for two, -7 for three, and -10 if all four are drained.
This is just a way to turn the BAD concept into an in-game penalty for circumventing the "action movie" logic of a boss fight. The PCs can still go right ahead and alpha strike the boss or otherwise try to cut off the head so the body will die, but it's not as easy if they don't play by the genre rules.