If you think that you can earn only 90 gems in total in Boss Rush you are wrong. In fact, you can earn an extra 10 to 90 gems if you are a good player and if you are lucky to find a partner as good as you and if the game does not crash during play. 10 to 50 extra gems will not pose much of a problem but to get 90 extra will depend on luck and partner. There is no cheating, hacking or any foul means to achieve this. Just use your grey matter to think this out. I am not giving the solution because I think you will be more proud of yourself if you can find the solution yourself. I just want the dedicated gamers to know of this possibility and work out the solution. I know many good players have finished the game and could see no more challenges left.
Additional info: I have dug deeper and found that you can earn up to 290 gems and I have done it. It's very difficult of course and over 200 gems will be of no problems to good players. Only game crashes will do you in.
Quick info: play Level 4 both entries first, solo and hard mode if you can finish it successfully.
Then in Multi mode play 3-4 and 3-4.
Then in Multi mode play 2-3-4 and again 2-3-4.
Then in Multi mode play 1-2-3-4 and repeat 1-2-3-4.
Save your game every time you finish level 4 successfully. When in multi level session, you or partner cannot quit the game until level 4 or you won't get the max gems possible. If game crashes in multi session (floors), then you are done for.
Weak players should start from the Level they can finish without revival and work backwards as above. Then hitch a ride with strong players for the higher levels.