Go after the capital ship, as they are very hard to replace, after they are over level 3 and 4. Most of the time, you can buy the capital ships, and by the level up to 3 or 4, but after that, it must when in some fights, in order to get any higher levels. Level 6 is abilities are very powerful and ther lose can break them in the long run. Also note that no matter which type of capital ship it is, it always has an abilitie or two(or three). So destroying one, not only removes those abilities, but also all that fire power and other bouns.