If he's playing true to fluff, he'll have flamers everywhere, so keep your Venoms at an appropriate distance.
He also may hide single heavy weapons in squads to take advantage of their reroll chapter tactic. Give 'em the Necrons treatment and wipe entire squads at a time.
Otherwise, bring all the Dissies. This may also be a good time to use Incubi if he's infantry heavy. Flayed SKull is also a good obsession to have as he'll likely plant Scouts in cover.
Yo ho, yo ho, a drug-fueled BDSM space-elf pirate's life for me!
Can I get a Roll Tide?