Board Thread:Questions and Answers/@comment-31479589-20190616100306/@comment-31479589-20190622143751

Shadowman19028 wrote: Upgrading legendary or epic requires about 3000 to 4000 shadow energy and that might not even get it past the rare stuff I'm using I once saw someone got an upgrade on their Shadowslayer, which was very underpowered and dealt next to no damage to Angel. The next thing happened was Angel got stomped in each round with only three Rolling Waves combo.

Legendary have higher upgrade value, they'll catch up with your Rare armor, unless the power difference is beyond 2000+. And you can buy shadow energy from the shop too. There's also Survival where you can grind for shadow energy or coins, which in turn can be used to purchase shadow energy. If you don't want to use shadow energy, you can try getting card stacks to upgrade an armor you already have. You can get them from Chests, Marathon, Season Rewards, Event packs (just try to complete at least one run in each mode, don't bother with the set if you already have them).

These, however, require lots of grinds. Epics have lesser upgrade value than Legendary, but still good enough to help you with main quests. They're cheaper and easier to get, making them better in the long run imo.