As anyone who's sunk their teeth into this dark fantasy manga knows, it's got quite a well-crafted setting. A war-torn setting where white-knuckle action is mixed with grim hardships borne simply out of human nature. The supernatural presence is subdued, but also hideously powerful- a monstrous evil that's simultaneously all too human. It's a great setting-but how do you adapt the awesome elements to a roleplaying game?
Step 1: Use Fantasy Craft. It's built so that magic is optional and not integral to the game's content, and has the crunch to really get into a detailed representation of the subject matter. I know no better system for the job.
Step 2: Don't tell them it's Berserk. To steal some advice from one of the people I brainstormed with on /tg/: Tell them it's 'Badass Mercenary Quest' or something. A little bit of foreshadowing with dark lore, one or two demons, more mercenary adventures, then OH GOD WHAT THE HELL WHY?! Which happens to be an excellent summary of step 3. . .
Step 3: Gradual, ceaseless evolution of the story. Escalation is already a natural part of most rpgs; what Berserk can teach us is technique. Characters get more powerful, yes; but it's not just their skills that develop. Their personalities change, new personality traits appearing and old ones fading away as they become a little more mature, a little wiser and more cunning. Their long-term priorities change, sometimes due to introspections, sometimes because the change is thrust upon them.
Step 4: Player-driven overarching plots. It's the changes in the protagonists that lead Berserk's story to evolve, rather than just having static characters who are coincidentally caught up in increasingly grand events. The adventures occur as characters pursue their own goals- to form a mercenary band, to hunt down monsters, to rescue someone they love.
Step 5: Unfair odds. Characters in Berserk are constantly faced with encounters that are in no sense of the word "balanced". A wolf pack. A hundred men (many with crossbows). A demon that smashes through foot-thick stone walls and tree trunks. In metagame terms. . .I once played in a game (the "Elysium Nebula" one I've mentioned a few times in the past) where the GM was increasingly open about the fact that he was putting us in bad situations without bothering to come up with a way we could survive them. That was our department. And because he was willing to sit down and go over the particulars of the situation with us, and rewarded innovative ideas (Say, hacking into an enemy's comm channel and impersonating their commander), the result was one of the best games I've ever been in. It also helps to let the PCs have a variety of interesting tools on hand- ropes and throwing knives and small explosives. . .
Step 6: Inventive action. Berserk is full of innovative and interesting fight scenes, where characters do far more than cross blades. Spycraft's myriad combat feats help with this, but what I like to do is have my NPCs start the snowball rolling; come up with interesting tactics based around the situation and any unique abilities they have, then give the players chances to mount unorthodox responses of their own. . .
Step 7: Life sucks. The world of Berserk is a terrible, terrible place to live- full of desperate people who, as in real life, are motivated by their desperation to do all sorts of horrible things to others. Make it clear to the players from the get-go what kind of setting this game will have; if don't understand that the game is *supposed* to feel harsh and merciless, they'll find the experience much more frustrating from metagame standpoint. Rude awakenings are fine, just spring them sooner rather than later.
Step 8: Any means necessary. The PCs are by no means an exception to the above statement about desperate people. In Berserk, Guts would have died ages ago were he not capable of taking extreme measures without missing a beat- measures that have, on more than one occasion, lead to a mutated monstrosity the sized of a house referring to him as a monster. I talk in steps 5 and 6 about encouraging/rewarding innovation, but in Berserk it's not always enough to just be clever. You have to be ruthless as well, doing whatever it takes to come out ahead.
These are the pieces of advice I've got to offer. Any further suggestions are welcome.