This boss hides in mirrors - he starts out with six. You can tell which mirror he's hiding in because it will be glowing pink. Yama jumps around from mirror to mirror - don't bother trying to catch him while he's doing this. Wait till he settles in one mirror, then keep following him until he pauses. Attack using Fomoire or any other Destroy element attack. The best time to attack the mirror is when it pauses (although be careful - a bunch of owls will shoot out of the mirror when it pauses - they're easy to avoid if you just step to one side). Another good time to attack is when Yama starts gathering energy to shoot a green ball at you (he will also pause for this). Get in front of him while he's still gathering energy and then attack. You have to catch him RIGHT at the start or else the green ball will hit you before you can attack. (Also, don't stand too close to him - if you use Formoire you won't have to anyway.) Be careful when the mirrors start turning around to attack you - run to the edges and wait until they start moving in a slow circle again.
You'll have to defeat every mirror before you can start battling Yama directly. Every time you break a mirror, Yama will rush out, so be ready with a Guard so you don't get hurt. Once all the mirrors are gone, there will be a short period where you have no mana. Run around until the red lights disappear, then use Beithir or Fraxinus to attack from a safe distance. When his blue aura disappears, use a Thunder Element. (Mabinogion is excellent for this.) Or when playing as Keats, use a Fire Element (Durandal is a beneficial one to use). When the lights turn red again, that means you won't be able to use your mana so run around until that disappears. At the end, it's a Mix absorb.
Yama is a unique Folklore as he is the only one to have shown human level intellect, proven by the fact that he talks, where as the other Folklore appear to be mindless beasts. He is also the only Folklore to have any form of authority, where as the other Folklore appear to be inhabitants.