Despite impressive firepower, Sentinels are lightly armored and are easy fights when you encounter them solo. When they are paired with Guardians, maneuver your battleships so they fire on the Sentinels first.
When you start discovering level 4 red stars, Cerberus ships become more dangerous. Interceptors are aggressive. When your ships enter the same sector as an Interceptor, it comes alive and starts hunting them. It will even pursue ships into different sectors of the map. The remaining ship types (Colossus, Destroyer, and Phoenix) require multiple battleships with many upgrades in place. The best way to take them out is to eliminate any smaller vessels first, then focus on the capital ships.