What are best units to use against

battleships or rrg. Or move all your arty close and send in troops to be fired on. Then move your arty in to fire and retreat. Repeat ...

A couple different options here. Rocket artillery makes for a a good counter to artillery. RRGs can out-range artillery and that is probably one of your best bets. I wouldn't advise you rely on this regularly but sometimes you can outmaneuver the enemy with speedy units and rush the artillery, especially when the enemy advances onto your terrain and has to slow down to 50% speed.