I think it would be either the Domain of Gunyun or the Slumbering Court. For both domains, players need a very strong shielding unit and a potent healer, especially in Slumbering Court because the Rifthounds can apply the "Corrosion" effect. The Domain of Gunyun has more annoying monsters, and the Ley Line Disorder prevents players from making things easier for themselves by inhibiting Pyro characters.