I did a little preliminary work with humans versus hydras. My standard experimental setting is a nasty place for the AI-controlled humans- they're got nowhere to run, and the hydra is a lot bigger than they are, so they spend nearly the entire fight on the ground due to getting charged and knocked over. I had to give the humans full iron armor, an iron battle axe and shield, and GM Fighter/Axeman/Shield User/Armor User/Dodger before even 40ish out of the 100 humans survived.
Nearly 100% of the survivors were missing one or more legs or feet. I suspect this may be because losing the ability to stand caused them to focus on fighting the hydra while on the ground, rather than wasting time repeatedly standing up and getting knocked back down again. The AI clearly does not know how to handle itself in these conditions; perhaps I should do some "personal" arena mode testing with one human-controlled adventurer at a time, counting the number of attacks it takes to down the hydra each time or some such.
Also, at least one hydra I looked at had already had wounds scar over within the few days of fighting. These guys heal fast.
EDIT: Did some one-on-one human-controlled combat, starting with an axeman. Going straight for decapitations isn't the best strategy; hydras are too large even for greataxes to score an instant kill. Instead, cripple at least two of their feet (so they fall over, giving them loads of combat penalties) and then go to town lumberjack-style on a given head or neck. It'll take a while, but it will come off. Once it does, dance around outside of attack range until the bleeding stops, then start in on the next neck. It should bleed to death within fairly short order.
Still haven't tested anything but axes. Going to try spears next.
DOUBLE EDIT: Spears are winrar. Strategy is to go for the upper and lower body as opportunities appear; lower body gets gut shots to make then nauseous and slow, upper body heart shots or ruining both lungs gets you kills. Both also sometimes get spine shots, which never hurt. High-skill (GM in useful things) armored spearman managed to take out five hydras in a row this way before the sixth got lucky and ripped his spear arm off at the elbow.
This leaves us with a use for all three of the weapon types: blunt weapons for armored combat, broad edged weapons for human-sized or similar unarmored combat (with any armor weaker than your axe not counting as armor), and spears for enormous unarmored targets like the various animal-style megabeasts, plus giants and similar.