Shoulder endurance/strength training

Hello,

My left shoulder has been getting heavy and tired during sparring especially when I do a long guard for extended periods.

What are effective ways to help build endurance/strength in the shoulder to keep my hands up especially for long guards and jabs?

Shadow boxing weight 1-2 lb dumbbells? Shadow boxing with the glove? Arm circle with weights? Shoulder presses with a barbell? Lateral raises with dumbbells?

What has worked for you?

Thank you in advance.