[Menschenfresser]

Since you are looking to knock out most of his fleet and counter attack, without, I assume, leaving your system completely devoid of ships, traditional ship to ship battle is not the way.

Granted, I've never actually fielded or fought such a fleet, but I'd go with an all out non-ship to ship offensive. Wear him down and scatter his fleet. First turn on the intel machine. Change it up all the time. Find out what works. I usually go for ship insurrection, because it seems that even in the face of large counter intel it works. I believe you said something about 200,000 points. That can field a counter intel project and three SIs.

Next I would send numerous cloaked, fast wolf packs, with enough solar collectors to stay in the field forever. Equip these with sun destoryers or plague bombs or standard weapons or mine layers. Cause havok wherever you can, no matter how small. Plague his planets. Attack small Groups of ships when you can (ie colony, transport, mine layers, etc.) More than likely he will take the time to replace these. This really isn't meant to knock out his capacity for building (that is for your main offensive) this is just to keep him moving and producing ships other than those for his main attack fleet.

Next you'll have to play George Washington and try to guess where his main fleet will strike. Set a star destroyer over the star of the system you think he will enter first. Preferably one with just asteroids or at worst a few of your own colonies. As soon as his death fleet enters "BOOM." He'll never know what happened, especially if he thinks he's entering a system with a few of your colonies. I know it's sick to vaporize one's own people, but don't the good of the many out weigh the good of the few.

Keep us updated as to how it comes out. And once it's over I'd really like to hear what strategies and tactics you employed.