I am an acrobat and I do alot of one arm holding skills. What I do that works best is weight training.

You can use a 20 lbs dumbell, medicince ball, 35 lb barbell (short Olympic bar bell), or any kind of weight that you can lift and hold safely above you head. Try to hold it above you arm with one arm for about 45 sec - 1 minute. Rest for 15 -20 seconds between sets. Do 3 sets.

Good luck!

Tell your flyer to loose weight lol just kidding, keep practicing holding it there, check your arm positioning and lift with your legs

I agree with the weight lifting it will do nothing but benefit you.

Make sure you are lifting with your legs, lock our your arm when the stunt hits the top, and hold your arm close to your head.

Attach a weight 5 lbs or more to a rope with a handle on the end of it lift the weight wrapping the rope around the handle this will strengthen you wrists tape your wrists and use a wrist band right below your elbow. Also to kepp doing chairs your stabilizer muscles that are important with co-ed stunting might did to develop more