It's the easiest to consistently fight the higher eras. I never lose a battle when attacking in PVP Arena in this city. I do not spend any medals on it, and I don't bother maximizing my number of fights, mainly because top 50 is not something I care that much about. The only top 50 reward worth extra effort is the 20% chance at 50 Diamonds, but I get between 200-500 Diamonds daily just from collecting my Diamond mine cities, so even if I was guaranteed the 50 Diamond reward every week I'm not sure it would be worth the effort. Maybe if it were my only active city, but it's not.