Your DM wants you to flee. I won't tell you all a Slaymaster can do, but it's CR 11, and a single Kython adult is CR 5. With next to no weaponry, only up to level 3 spells, you're SOL. Your best option if to get the Kythons and Undead to fight each other. They Kython's should be able to take care of a fair number of Undead, and the Undead should be able to take care of a good number of Kython's. This will reduce the numbers greatly, and you'll only have to fight one force of considerably weaker forces, which is a good thing.

Kython's are intelligent, after all, so they'll probably figure the undead are the bigger threat (Slaughterkings are drastically more intelligent than Humans).

My definative recommendation - Let them kill each other. You will die, unless the DM is very nice to you. Perhaps get all the villagers very good at Hiding and let the town get decimated in the Kython/Undead battle. Once they're done with each other, spring up and kill the weakened forces.

The other option is to craft a whole lot of Wands of Fireball, and nuke the undead forces - Kythons are demonbred, so most likely resistant if not immune to Fire, though. Rely on Scouts (Complete Adventurer) to deliver Hit and Run tactics to the Kythons, and they should eventually be killed. It woud take a while, though...

Best tactic I see is fleeing, or letting the armies weaken each other first.