Leadership is really hard to grind in the early game, but once you hit mid to late game the best way it to farm 2 star and 3 star versions of past SOL subchapters. You get yourself closer to 4 starring every SOL sub chapter (Idi), you get a bunch of xp, and if lots o leadership. Right now, your best bet is just level up cats, you get 1 leadership every time you hit a certain user rank