Boasting high range, Battleships (sometimes shortened to BShips), are powerful, if expensive, Indirect Combat attackers.
Battleships have the highest possible Maximum Range of all the Indirect Combat units capable of hitting Land Units. Battleships also do considerably high damage to almost any unit it can hit, and can deal significant damage to Mega Tanks. Thanks to their very long range, Battleships are excellent area denial units, covering a wide area, while also being very difficult to approach without Air Units thanks to Sea tiles blocking any advances.
That said, Battleships alone are defenseless against Air units, and should be protected from B-Copters and Bombers with B-Copters, Anti-Airs, Cruisers, and potentially Fighters or Missiles. Battleships are also incapable of hitting a submerged Sub, so caution is advised when advancing near enemy Ports. If your Battleship moves within firing range of the enemy Port before they build a Sub, however, you can effectively neutralize this option with just a Battleship. Otherwise, if Subs are proving to be a threat, a Cruiser escort will be required.
Due to their high cost and slow movement, Battleships are difficult to deploy in an optimal position, but can easily force the enemy back if deployed in a time or place that an enemy does not expect.
Once deployed, Battleships are very difficult units to play around with their high range. It may be better to factor in enemy funds to determine if they will build a battleship; if an opponent is ending their turn with 10000+ funds available, they may be saving for a Battleship should there be a port relatively close to the front lines.
Air units are generally the safest option for taking on enemy Battleships; if not properly defended, any Air units capable of attacking Sea units can deal considerable damage to a Battleship which, if not destroying them, will lower their combat effectiveness severely. Opponents may be hesitant to repair a Battleship right away as it is a very expensive unit to repair, so it may be able to whittle it down over a span of turns.
If you are unable to threaten a Battleship, it will push your forces back very far inland, forcing you to give up vital ground and properties in the process. The best course of action to deal with an enemy Battleship is to deny the enemy the ability to feasibly build one, either by pressuring them into buying other units or to focus on areas that the Battleship is not able to reach quickly.