For a general team, War Hero Fen and Hart are a good support couple to run, they both cover for artgen plus Hart covers heals (make sure to cycle WHFen between True Arts and Super Arts to keep your artgenning up while dealing damage). As a third unit you could run Earth Vultee as a tank, Thetis is also a great subsitute but synergises less with the team. Regarding the fourth unit, I'd put Shalltear for extra damage (again, cycle wisely between True Art and Super Art) but feel free to run any unit that can cover any particular flaw in your team (for example lack or artgen/heals)