DON'T use pipe pliers, mole grips, or any form of adjustable spanner! You will simply disintegrate the bolt head and find it hard to put back securely.
Forget the plastic spanner, they are worse than useless. You need to get a correct sized socket and whats commonly know as a 'Crack Bar' but really called a socket extension bar. This is a bar that you can fit a socket on the end of, (preferably 1/2" fitting), then, because the bar is usually about 18"-24" long, it gives you greater leverage. You will need to stop the blade rotating while you try to undo it so grip the blade, or get someone else to grip it, wearing a pair of tough gloves, sometimes known as rigger gloves but often made from a tough canvas. Apply pressure gently but firmly, increasing the pressure slowly. Do not try to jerk the bar or you may injure yourself/someone and not do the bolt any good. Patience and concentration will eventually win and hopefully with no injuries.