Over on The Mule Abides there is an excellent discussion on anti-climactic "boss" fights. Specifically, many-versus-one "boss" fights.
Many versus one doesn't work so well in GURPS, either, unless you're one of the many.
Why is that? A few reasons. Amongst them:
Realism. In any system, being outnumbered isn't helpful in reality. Do you know anyone who's been in a multi-opponent fight who'd do it again by choice? Especially if they had the choice of outnumbering their foes? Even SEAL teams come in with overwhelming firepower, they don't decide that they're better than their foes so what the hell, let's give them odds. No way.
Players know the odds. It might not be a problem in a medium like comics or in a kung-fu movie, but it's a big problem in games. PCs don't take their turns fighting the boss monsters or main kung fu master or super villain one on one. They know how the story turns out - they lose one by one, and then they realize they need to work together and then they do and then they win, hurrah! - and skip right to that part. They mass up and jump the guy. They take out their biggest weapons and smash the bad guy as hard as they can immediately. And they sure as hell save up something special for him, so even resource-depleted they have something to whack the boss with.
Rules. For example, in GURPS, you roll to hit and then the defender rolls to defend himself. Multiple defenses generally are at cumulative penalties. So a large number of attacks increases the odds that later attacks get through - and what's an easy way to get a large number of attacks? Many-on-one. Beats (from GURPS Martial Arts) transfer from one person to another. You can grapple an arm and preventing retreating or parrying. Blocking spells are limited. Etc. Outnumbered is bad, unless you vastly out-gun the opposition or are using genre rules to force one-on-one fights or weak mooks, and even then, that doesn't help NPCs any.
So what can an outnumbered foe do? You want the boss orc or the head wizard or evil high priest (the dreaded EHP) or the dragon or whatever to put up a good fight.
From the point of view of the
Fight a fight you can win. If you can't win, run. Don't even enter a fight you can't win unless you have to. If the PCs run down the EHP after killing his guards, banishing his demons, and dodging his traps, and corner the guy, he should die miserably. But don't fight outnumbered to see if you can win. A PC wouldn't do that, what's your problem? How did you get to be boss orc by fighting outnumbered and against powerful foes?
Don't fight alone. Why is the boss orc or head wizard or evil high priest outnumbered, anyway? Bring friends. Have guards, summoned monsters, clones or simulacra, and allies of all kinds. Don't be outnumbered unless you can't help it. Don't spare them, either - an evil foe should be sacrificing henchmen and flunkies and fodder as needed. Spend them like arrows as long as it gets the job done.
So have henchmen, or be multiple opponents. A dragon with five different attacks, attack immunities (to limit its exposure to damage), high DR, and piles of high defenses can help mitigate the effects of being outnumbered, but it's even better to be multiples. Use spells to duplicate yourself. Be a hive mind. Put up mirror images that have to be uncovered and dealt with. PCs know these things exist, but a kagemusha isn't any less annoying to deal with because you know one isn't real.
Don't be a pile of HP. This is huge. If all the big boss consists of is a pile of HP with good defenses and some DR (or low AC and a high level in a D&D clone), the big boss is just going to get whittled down. Just like everything else. Ten gnolls have more HP in GURPS DF than one gnoll chief, and probably take more killing (and get 5-10x as many attacks). If a wizard is hard to hit but dies when hit, whoop-dee-doo, he's just as hard to deal with as a stirge (hard to hit, dies when hit). In D&D clones you can give them more HP but it's just a question of whittling them down until you win. Boring. But if you've secreted your soul in an object and can't die until it's destroyed, well, now it's an interesting fight. If the Evil Wind God is unkillable but destroying all five of the summoning stones sends him home, and the PCs don't know that coming in (but could have found out, or have skills to check for hints, etc.) it's an interesting fight. It won't just be settled by the Knights and Barbarians doing 3d+whatever cutting damage and dropping it. If you're mortal and just hard to hit or take a lot of damage, you're just bigger fodder. Sean "Dr Kromm" Punch wrote a good post about this a while back in a whole thread on this subject.
Don't fight fair. Narrow cliffs so people have to fight one on one, trapped rooms, reinforcements, anti-magic spheres, whatever - don't even remotely fight fair. Fight like a PC would if they had your stuff and your smarts.
In my experience in GURPS, many-vs.-one fights meant a dead "one" pretty quickly. So my enemies stopped fighting by themselves, or at all if they couldn't hope to win. I recommend the same!