A couple people pointed out that in Swarm Battles, if your character is able to bust through 20 guys throughout getting hurt, it's okay. Well, I had mused about this, but…I mean, could you classify that as a battle? Maybe, but certainly not a Swarm Battle. That's more of a 'I'm trying to walk here, but these guys are in my way…I guess I'll have to flick my wrist and kick their asses' kind of thing, if your protagonist/antagonist is someone like a 30-foot giant who shoots fire out of his nipples…or perhaps that's just my new character… If you want to do this to show the supremacy of your character, by all means, do, but it wouldn't fall into the battle category, per se.
And onto 10!
Climactic Fight: A fight that has been anticipated since near the beginning of the story itself. Involves the BIG BAD GUY (BBG) and the Hero, or a combination of BBG and his cohorts/Hero and his cohorts.
Riser Fight: Fight involving the Hero/Hero cohorts and one or two of the BBG's more powerful henchmen. Usually, the henchmen are trying to stop the Hero(es) from reaching BBG.
Don't get these two confused. The Climactic Fight is the fight. There can be several Riser Fights and lesser ranking fights, but just one big Climactic Fight is the key. The bottom line: it all comes down to the Climactic Fight.
There's how you approach it. Now, how do you write it?
You've got to think realistically, for starters. Since this is the big fight, it'll be tense. Usually before things like this you see a lot of flashbacks as to why the hero went on the quest in the first place, who the BBG killed, etc… You can do this if you like—it can reinforce the character's feelings toward the enemy. I, personally, don't recommend it. When I see it, I don't like reading it.
Dialogue will be a focal point. Conversation before and during a fight like this can be crucial. You'll want the Hero(es) to talk to the BBG, maybe ask him 'Why?', or maybe he can taunt them or bellow with rage. You could also have the BBG do a monologue, as is characteristic of most big enemies—just make sure it doesn't run on too long.
Fights like this are also opportune times to pull out the ever popular Blood-Rush Theory. I mean, it's the climax. If you have several characters you could have one of them slain, if you wish. Or a single character can be at death's door when in drops the dues ex machina…the possibilities are endless.
When writing it, it's a good idea to have the fight start with the BBG with the upper hand. Maybe he's got some lasers shooting around, or tons of henchmen assaulting the Hero while he tries to get him, or something. You'll want the tables to turn slowly; you want the reader to think, 'Damn…can he beat the BBG?' and have them contemplate that the Hero may actually lose.
If your plan is to have the Hero lose, then make him lose, I guess. I'm just gonna go out on a limb here and say that you want him to be victorious.
After the fight, if the Hero didn't kill him outright, maybe have the BBG reveal something in his death throes. It could be something like a personal truth, or you could go Star Wars twist-style, or maybe he could tell him that there's a bomb in the building/place and he has to haul ass before he's blown to hell. That's another door with endless possibilities.
You DO want the hero to get hurt. Endgame. Even more so than Swarm Battles. The Hero's struggle to win has to be VERY, VERY pronounced. And if your BBG is a total dick, and has really screwed with your Hero, and the reader now hates his guts, then make his death slow, painful, and pronounced. Then the reader can think to himself, 'yes, take that, you douchebag.' It's quite effective.
-This is the fight. Remember that.
-Dialogue is a good thing. You don't want to go overboard, but there should be plenty. Taunting, screaming, monologue…
-Whip out the Blood-Rush Theory if you want. Have one of your characters die, or if you have only one, put him on the edge of death and have him be rescued.
-Try to have the BBG winning, winning, and then SLOWLY let him be brought down. When he does bite the big one, you can either choose to have him just die or choke out a final message.
-When the Hero takes a big hit, express it eloquently. When the BBG is taking hits, you want to be just as descriptive.
And that's all for that. Good luck to ya!