Board Thread:Questions and Answers/@comment-38784665-20190312015051/@comment-31479589-20190312023803

Ideally, you should complete all the remaining Tournaments, Challenges, and Survival (two times, with and without Eclipse) first. Save the boss for last. You don't even need to fully/max upgrade all your equipment, it's possible to defeat them even with half-upgraded items. Max-upgraded items only make the fights much easier.

Completing all the leftovers will grants you with all the possible coins you could have, therefore allowing to upgrade your items.