I had to replace a coil in situ and used a self-locking oil filter wrench to hold the pulley, levered against the body it would tighten its grip as I undid the bolt, freeing up both hands for the job (breaker bar).

Or a strap wrench, or right angle impact wrench as there is not much room down there.

Recently replaced whole compressor (variable displacement Calsonic) with a reconditioned one, and had to swap the pulleys (different # grooves) on the bench - in both cases I held pulley in my hand and an 18v impact driver did the job in a second...