I have been stuck on the Fight for Earth mission in Nexus for WEEKS. Nothing I do works. I've parked my fleet kilometers away from the Entity gate, the better to pound the daylights out of whatever pops in before their shields come up.
If this is the mission I think it is, the one that's burned in my memory from the number of times it's kicked my space-butt, then I'll share my strategy:
Lots of anti-armor artillery missiles. I can't remember what they're called specifically, but load up on them.
Memorize the timing of the enemy ships entering through the wormhole.
Memorize the amount of time it takes your missiles to get to the wormhole.
Force-fire before the enemies arrive so that your nukes hit right after they arrive but before their shields go up. What doesn't die, should be weak enough to mop up.
If you're speaking of a different mission, or this tactic doesn't work, then idk.