A ring spanner of the size to match the locknut can be easier than an open ender especially when the gland is close to a screw boss. (Slip over the cable end with the power off/isolated). Worst case you get someone to hold the gland side with a spanner and you knock the locknut round / tight with a sharp, flat bladed screwdriver and a hammer. The steel screwdriver tip digs into the brass etc. Last report that really, ideally space your glands so you can get a spanner on.