Don't tell where you learned this, but you can take a pair of needle nose pliers, held vertically, stick the points in the slots and take the head on or off.  I probably have a spare wrench or two.  I'll check and PM you.

A pair of heavy duty snap-ring pliers will also work, and let you get better leverage / control than a pair of needle nose pliers held vertically.

Visa grips held horizontally will suffice.