They provide rewards for each fight. I have been just doing the highest tier and getting the rewards for it. It's a small amount each time but it adds up.
It's pretty tedious, but as @Ayden_noah1 said, the rewards add up. Find the highest you can do, with a solid team, slap on some boosts, and just run it on repeat
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But hurry up cause it's pretty tedious
After so many weeks I know
Kabam why don't take out that 100% completed, so people would play again?