This battle is tough. Here's a description of what I needed to do to beat him:
Do as much damage as you can right away before you and Jiren transform. If you miss one of your first attacks, just retry the battle. You are going to want to maintain a steady health advantage over Jiren.
After you transform, your health becomes maxed once again, but you should try and preserve that and your energy. You need to take out Jiren ASAP before he transforms. His base form is easier if you maintain your distance and spam blasts.
After Jiren transforms, he will have full health once again. If your health is low at this point, just retry the battle. Also, you are going to want to have a lot of energy charged up to use it on Full Power Jiren. He will get in your face so you need to be ready to blast him away.
Ultra Barrage requires 4 energy bars while Ultra Kamehameha only requires 3. I would stick to Ultra Kamehameha.
Ultra Movement is a very good way to send Jiren flying when he gets close to you, but it requires 2 skill points. Because of that, it may be better to send him flying and then use two Ultra Kamehamehas rather than going into Sparking Mode.
Even after all this, I was on my last health bar, but I won! Give it a shot.
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