That’s also what I am doing, this is what I do:
1- Farm event missions (Prioritize this first as events will be over in one week)
2-Reach at least Rank 40 in PvP (You will get gems every 10 rank and for reaching there the first time)
3-Do all challenges in Events.
4-In the “Upgrade Tab” on the event, you get 40 gems for clearing with only the specified character. (so instead of using 3 character just use that single one)
5-The new Broly and Vegeta event missions give a lot of gems, also farming the equipment specified in the mission will give you 50 gems.
*Do the vegeta first as it has a deadline.
6-After all that is done then you can do all the challenges in the story mode to get all the stars and you get 3 gems per challenge.
*Make sure you do PvP because after the season is over you get 2000-2500 gems.
If you have 8$ to spare, you can buy the Robot that give you 40 gems every 20 hours for a month. (Recommended)
*Always use different characters when doing story mode as you gain friendship level and that’s 2 gems per level.
*log in everyday
*dont use gems on useless stuff (Just the daily discounted summon “20 gems” and maybe two refills of energy “10 gems”)