Can I win Deadman's Dike by using O Elbereth Gilthoniel on Thaurdir? Or was he intended to never leave play. It seams like a fool proof method to having a good chance at this quest

O Elbereth received errata in the most recent FAQ. It doesn't work vs. unique enemies:

"O Elbereth! Gilthoniel! D 132 Title should read: “A Elbereth! Gilthoniel!” Should read: “Response: After a non-unique enemy attacks you..."

I knew it was too good to be true.. That card was so good on Hobbit quests too. Thanks