My co-op partner did every side quest, and now our main fights end before I can try anything

I'd practiced a new attack combination. When we reached the next boss, my partner defeated it before I finished setting it up. "Those side quests paid off," he said.

We play the main campaign together, but we also use our characters between sessions. He likes completing the map. I like learning a fight with the gear I have. I knew he'd been exploring; I hadn't realized how far his character had moved beyond mine.

In this game, we can equip older gear, although that won't necessarily remove every advantage. I asked him to do that for the next fight. He said it felt strange to earn upgrades and then pretend he hadn't. I understand that. I also don't want to become the spectator in the campaign we agreed to share.

Would you agree on a rough strength range for shared missions, try a different difficulty if the game allows it, or stop trying to use these same characters together? I want his extra play to be welcome without deciding how our whole evening unfolds.