Last night I defended against Dark Angels and Daemons. For this game we did the second Planetstrike mission. Instead of a Helm's Deep type defense, I put my bastions spread out, with one in the middle, and one on each table edge, slightly off-set so that each bastion could see all the way to the other table edge. I then put my cheapest troops (not easy with Eldar) inside the bastions because I knew they were going to take Flamers and just burn my troops inside.
Inside the bastions I had two units of 5 Rangers, two units of 10 Guardians, and in the center a Vibro-Cannon unit. Each unit is about 100 pts, which isn't too cheap but was the best I could do.
The rest of my army was in reserve. I had a Seer council, 6 bikes, a Falcon full of Fire Dragons, a Fire Prism, and 10 Dire Avengers in a Wave Serpent. For the game, I basically flew around turbo-boosting as needed and frying things as needed. This more mobile defense worked out great. Since I only had to claim one objective, I could distract them and pull them far away from the objective I intended to claim, and then turbo-boost into position at the end of the game. I killed very few things, I don't think I killed a single Dark Angel model. I took out as many flamers as I could since they're fast, and I killed a Soul Grinder with a lucky Bright Lance shot, but that was about it. This game was won through resilience and speed.
So for a mobile army like Eldar, try a spread out defense. You force your slower foes to commit, and then you can rapidly redeploy as you see fit. Remember though, in Planetstrike you have to totally clear the objectives. None of this last minute contesting if you are defender.