here are some of the best exercises to help improve your armwrestling, other than obviously armwrestling itself. These are really good especially if you don't have a table like myself. These are only a few, I will give you more:
preacher curl, weighted dips, weighted pull ups or chin ups, shoulder press, close grip bench and working with CoC's or heavy grips is one of the best.
note: on the bar extension hold, make sure the small end of the bar is not resting on your wrist or forearm. it looks like it is on mine, but if you look closely, it isn't. I also didn't do the full amount of reps that should be done, I only did like 3 or 4 per exercise just so the vid wouldn't be so long.