My LG Rumor had some water damage on the battery. Now it says "battery is required...?". Does anybody know if I can fix this by just buying a new battery or do I have to get a completely new phone?

Get a bowl of uncooked rice grains and put your phone and battery (unconnected from your phone) burried in there for a day or two because the rice will absorb any moisture out of the phone and/or battery.

Go back to the store where you purchased the phone and have them check the battery for you. They will do this for free. They will also be able to determine whether you just need a battery or you need a new phone. The final thing to check with them is where you stand with your contract. It could be that you can get a new phone at no cost by extending your contract.