The cowardly part is because all of the enemies in one player mode (equivalent to offline) have much lower stats than in multimode. The bosses are normally significantly weaker in one player mode than in multimode, like only about half as much hp (olga form 1 offline 12,500 hp, online 23,000 hp, olga form 2 offline 16,000 hp, online 27,000 hp), and weaker attacks, lower defense, lower resistances, etc.
Nothing is stopping you from hunting it by yourself without the aid of others. You can still play alone in a multimode game without trying to exploit the system by fighting weaker enemies and weaker boss in one-player mode for, of all things, a pgf, that's the point.