bad advice imo as they are THE fastest 40k army out there, and they have lance weaponry so armour isnt the best idea.try using heavy vechiles like land radiers and try the flanking manouvare it might work if you outflank them good luck
could you give more details of his dark eldar army?? and they dont have that much ap2 imo unless he is cheating.
i use drop podding dreads with heavy flamers and assault cannons and it goes a dream, also try whirlwinds
hope this helps