I don't think the people complaining of losing gems realize how many resources the game gives the player to help win the battles.
You only lose the gem if you drain during the gem battle. Therefore, set as much up to prevent draining during the battle:
- You can make the secret skillshot or shoot combos to get portal locks. Use the portal MB wisely, controlled play trap and shoot.. treat the extra ball(s) as a spare and not something to get points more quickly.
- You can bring in Thor or IMMB into the battle for additional balls and ballsaver. Not only that, both Thor and IMMB come with TWO add-a-balls (Antman mystery and Eye of whatever at Sanctum).
- If you get the time gem early, all those ballsavers last longer, making later gems easier
- If you need even MORE ballsaver, knock some drops down, figure out where Super Ballsaver is on the grid, then bring it in during a Battle
- During the battles, skip the additional "extra points" shots and just go for the gem goal (e.g. mind gem - shoot the fast blink shot then wheel 3 times for quick finish, ignore the non controlled avenger shots)
- Remember Marksman hawkeye will give back a gem if needed
If you do all this, you should get gems pretty regularly. And remember, the comics and movies would be pretty boring if the Avengers consistently wiped the floor with Thanos and won every battle before the finale...