I used an ability to kill a monster. Another player used Wandering Monster, and then played Transferral Potion to move the monster to himself. They beat it. Who gets the level and Treasures?

If you automatically killed the only monster, combat ended and this whole sequence can't happen. If there were other monsters still fighting, however, this sequence is legal. You get the level and Treasures from the monster you killed after the transferred combat has been resolved. If the other player also wins, you draw Treasures first.