What's the best approach, whack it with a hammer?

I've had good luck flooding seized padlocks with wd-40. And before all the
wd-40 flamers chime in, yes, I did follow that up with silicone spray after

A can of Liquid Wrench or PB Blaster from your local auto parts store
will be far superior to WD-40. WD-40 ain't bad but there is better
stuff available.