I have two arthritic shoulders, and an artificial right hip.
Being right handed, I used to cock my 150 lb Horton with my right foot in the stirrup, bent over, grasped the string and then pulled while straightening my back.
With the hip replacement I had to switch to my left foot, and the job became tougher, but I used this method for 3 years before buying a rope cocker.
I expect the Exomag to be tougher than the Horton (I haven't shot it yet), because it has recurve limbs, and you're holding the maximum weight at full draw rather than at a let-off value.
I know I haven't helped much, but its probably worth it to get together with someone familiar with cocking a crossbow, and work with him on technique. It may be as simple as that.