As with all bosses, Viscount Vilhelm can attack normally, but also has an AoE that hits everyone twice. He can also heal himself for +5000 hp.
A similar strategy to previous bosses should work well enough: tankers in the front, and a healer in the back. He causes fewer problems than Battle Conqueror or Toxico Spider.
Viscount Vilhelm is of the Ice element, and therefore weak against Fire, and strong against Water, Plant and Flying.