Pin wrench the correct size will unscrew it easily in a minute or two. Won't be corroded with all that grease.
Unscrew & remove the greaser unit first so you can work
Dental pick set is cheap
on ebay and handy for all sorts of things including removing gland packing, oil seals
Pair of hide-padded curved-jaw mole grips would have done that job in the time it took to type this... don't always need exactly the correct tool for everything