]I'm going to attempt to guess his army blindly based solely on the thread title. It helps that I have seen me some Blood Angels). I am going to assume that you have a archetypical Dark Eldar force
Mephiston
3 Sanguinary Priests
1 Dreadnought (Librarian or Furioso)
3 Assault Marine squads in Razorbacks
2 Baal Predator
1 Big Assault Squad in a Stormraven (Let's say Assault Terminators)
Oh, hey, I searched for your posts and found his actual lists in this thread! I was darn close.
So, I was all geared up to tell you that initial positioning is very key to fighting mobile armies, and that you should maximize the time available to you in order to shoot at the best targets with all of your weapons, but two Drop Pods changes things a little bit. You still want to keep your army on the move most of the time. You're not going to win a straight-up assault with a bunch of roided up space vampires. That being said, sacrifices will have to be made- speedbumps to contain parts of the enemy army. Wyches are the right rool for the job, as they get to dodge and take Haywire, things that are able to stop Mephiston and Dreadnought equally. They can't survive 30 S4 attacks from Assault Marines, but they can survive 6 S6 or S10. If something Deep Strikes into your deployment zone, you need to either pounce on it immediately or else flee in such a way that the majority of his army can't just jump you anyways.
So, you avoid being tabled by skimming around, avoiding straight-up brawls, and occasionally tossing a unit to the wolves (angels, whatever). I would say priority 1 is kill the dang Baal Predator, as it will be in your face anyways and might even let you Turn 1 haywire assault it. Protect your Scourges from its guns, then leap out and blast it. Pounce on the Dread with haywire (blaster or otherwise) when it drops in. Next would be to focus down the most threatening of his foot soldiers, this should be the Assault Squads first (as they might not all have Feel No Pain) then the Death Company (as they are tougher, but as less important as they do not score). Try and cap some objectives, and hopefully laugh it up.
I know from experience and frustration that 30 Marine bodies are hard to kill when they move 12" and have some decent support behind them. But, it is possible. Know when to send one unit to its death and flee with the rest, hang on until the bitter end. And that's my advice for you.