[Strategia_In_Ultima]

Star destroyers are good if you want to use a temporary scorched earth tactic, you'll lose any colonies/ships/units in that system and so wiill they, but the planets are still present as asteroids, and when the war is going better you could make them back into planets again.

A Nebula can be handy in all three types it can appear;
-Sensor interference nebulae can provide you with a spot to calmly construct your assault forces without them noticing, and if you use sensors and they don't, you'll be able to ambush them if they try another attack through that system.
-Combat Sensor interference nebulae can be handy if they use Combat Sensors, however your own to-hit chances will be less as well, so I'd advise mainly seeker-based and kamikaze-based (ship/fighter/drone, or mine) defenses, provided they don't deploy too many PD ships.
-Shield interference nebulae are excellent if they use lots of shields and you don't. If you use only armor on your ships, and they use mainly shields, you'll quickly gain a quite substantial tactical advantage, as you'll be able to devote more kT per ship to weapons or armor, while they would have at least several shield generators that don't work.

Black holes are also pretty good, they provide you with an excellent line of defence against the enemy, and it slows them down. What you could also do, is lay the maximum allowed number of mines on ALL warp points in the system, so that when they enter they'll be severely damaged if not destroyed, and if they're damaged there's a good chance the black hole will crush them, though I don't know if mines are subject to black holes too?

I didn't know nebulae creators were more expensive than sun nukes?