This is really annoying. I had a 2x Longshot mission and I carefully calculated my power level so that the enemy would be more than 2x my power in a prize fight I won, but it didn't complete the mission! Thinking maybe the bug is because my fighters evolved after the fight, I tried rift battles. I put my battle score on the line and took a huge risk by winning three rift fights with the enemy more than 2x my score, and I still didn't complete the mission! Please look into the Longshot mission code. I suspect it calculates based on your team's power after they level up, and also it doesn't work for rift battles.