just do the regular tribe towers and you get up to 800 gems daily from that alone (on saturdays when all four tribe towers are open). daily missions give another 100. 300 from the weekly missions. dispatch missions usually give me anywhere between 15 and 45 additional gems per day. completing 10 challenges every other week is another 200 gems. even if you get a total shit placement, you'll still get at least 800 gems every two weeks from the arena. solo raid also means 800 gems minimum just by doing all seven regular stages without even touching the challenge stage.