I'm looking at ways to improve my shoulder endurance. While sparring, I find that my shoulders fatigue alot faster than my legs and conditioning. I am trying to add some additional exercises in my routine. This is what I am considering:

1. Shadowboxing/hitting the bag with 18oz gloves

2. Interval punching type of drills on heavy bag

3. high reps of light dumbell exercises like lateral raises/front raises etc. (around 20 reps, maybe 5 sets?)

or would it be better to try to hold a medicine ball in front of you while moving around the ring for 2 minutes? (or something similar, with a time duration instead of sets/reps??)

Any other good drills? And how often per week would you do these in additon to your regular training?