How to Remove Burned-on Grease from Stove
I'd try dish soap first with a scrubby sponge. Then, a cleaner like Formula 409 with a scrubby sponge if that doesn't work -- this usually works with a bit of elbow grease/a few tries. 

And if that doesn't work, you can always go nuclear with something Easy Off Oven Cleaner (This is inconsistent with the manufacturer instructions, so I would not recommend it even though I've done it before and it worked).