Each faction to me seems to balance to other. By balance, they have a similar strength.
Humans/skeletons: Cannon fodder. The humans are regulare minifigs. Just raid your own collection for raw recruits. They are have been lots of castle, ninja, pirates and vikings. Plus use all your blank torsos. Sure, not everyone has armour and alot of guys will have to use crude clubs, but not everyone can afford armour and weapons. Then you have skeletons. I've read in a different post, you can use PaB to buy skeletons for $0.62. Or just use a skeleton head on a regular minifig.
Dwarves/Orcs: Strongmen and rare. Dwarves and orcs are typically portrayed as strong, hearty races. To make orc minifigs, you need orc heads (or similar heads, Killer Croc comes to mind), which aren't in many sets. Dwarves, short legs and beards, also not many available.
So who to put with elves? Elves are typically portrayed as agile with magic. I think vampires would be a good pairing. The elves have bows to shoot wooden arrows at the vampires. Vampires are a type of undead, making them a potential ally for the skeletons.