impact with a flexible joint is easiest to spin the nut off. I also used a Ridgid strap wrench to hold the pulley still (home depot brand, consensus best strap wrench on the plumbing forums
).
The tough part is actually getting the pulley off of the keyed shaft. I went to Autozone and rented an A/C compressor pulley puller. It is a tight space, but eventually worked. Almost impossible to avoid scratching the pulley and engine bay, it's so damn cramped and pulley pullers are rarely small.