If you don't have access to acetylene or mapp gas outfit, or impact wrench/compressor, try this.
Use short block of wood to stop blade from moving. Insert between blade and discharge hole as you fit long box ent wrench over nut. Be certain you have coreect fit !!! Then use heavy shop happer to rap on wrench repeatedly. That will eventually start it turning. If that fails, buy nut cracker at local auto parts store or online. That will split the nut and it will easily come off. Then wire brush the threads on shaft, apply bit of grease to them, torque new nut in place. Avoid reassembling parts like this dry. A bit of lube will prevent future headaches. Most mower mfrs assemble parts other then internal engine parts dry. Leads to corrosion issues over time. Deck spiindles should always be lubed.