Blaze a strategy I like to use is to close all of the warp points going into the target system. Then when I'm done conquering all of those planets I open the warp back up into the next system and then close all the warps in that system etc. This way you effectively attack one part of an empire at a time and can use your whole fleet against a small percentage of theirs. Just research stellar manipulation a couple of times to get warp open and close components
