Well the Webway portal with Wraithguard is a pretty well respected tool for ending super heavies that might be plaguing your battlefield. Its a respected tool for ending almost everything.
The Other synergy that is very strong is the Realspace formation wghen it is used to transport the Eldar Striking Scorpions from an Aspect Host Formation, with Aethersails. This works with Grotesques obviously, but giving the Scorpions an assault vehicle is like a dream come true.
Dark Eldar should be pressuring the enemy at all times and controlling the board. They are too brittle to truly stand back and shoot very effectively, even though their shooting is certainly effective. They need to keep enemies OFF their grass and keep growing more grass, so to speak.
anything that pressures and traps the enemy RIGHT where they started is a big help to you.
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Hold out bait to entice the enemy. Feign disorder, and then crush him.
-Sun Tzu, the Art of War