[Bruno]

It always depends on party composition and setup, but some rough guidelines:

Melee characters usually have access to good cutting attacks (which affect skeletons normally), while crushing attacks do really well, so martial artists (or really anyone who can swing a punch) tend to love skeletons.

For a four person group with the classic Knight - Cleric - Thief - Wizard

It's important to address the Cleric first, as they can change the entire combat situation with skeletons.

If your Cleric isn't the kind who uses the Turning ability or the Turn Zombie spell (and doesn't have Contingency Casting in his bag of tricks), he's likely still an able melee combatant, and can probably kill a skeleton every other turn, rather than merely cripple one. If he wields a mace or morning star (a good choice for smashing the undead!) he'll be in his home element. If he's swinging a pick, he'll probably be very unhappy.

If your Cleric is the kind that zorches the undead, they may render almost any number of melee skeletons pointless; just stand them off at a distance with Turning while your allies lazily throw rocks at them. If the skeletons have bows, this is more touchy! However, Turn Zombie can finish off (or rout!) a lot of skeletons in its Area of effect! the 4 second casting time means that you'll be looking at least at 4 seconds of fight before he pops that, so keep that in mind.

The knight is probably going to be plowing through skeletons like they're popcorn - if he's got a crushing attack, I'd expect him to kill basically a skeleton on the first hit. So if you want a fight to last five or six seconds, he needs at least five or six skeletons to himself. If he likes to make Rapid Strikes or Extra Attacks (as many Knights do), you could be looking at twice that.

If the wizard has a staff, they have a swing crushing attack - excellent against skeletons! If they have spells like Flame Jet, they're going to be in good shape mixing it up in melee anyways. I'd expect them to be crippling a skeleton every other turn, or doing very well "mopping up" skeletons that took a lot of damage from the Knight but survived (one per turn).
If they are the explody-spell type of wizard, all the elemental spells do damage nicely to skeletons, so that suit him too. This could potentially be a lot of skeletons in one big burst, but then he'd be either out of the fight, or back to poking with his weapon.

The Thief is in the most painful place because many thieves rely on stabbing things, and Skeletons don't respect that. Stick to the swing cutting attacks on his blades, if they have them. A knife wielding thief may actually be better off "backstabbing" with their Brawling skill of 15 and a boot to a skeletons backside!
Due to the Thief's combat effectiveness being so dependent on positioning, he's probably stuck with taking down a skeleton every two or three turns if he has a bunch of Expert Backstabbing. If he doesn't, not so good.