Play raids,dimensions,speed raids for gems,raids for boss rush tickets or you can collect gems and convert them into boss rush tickets in boss rush shop,1 boss rush ticket=200 gems,that's it.
The person below replied to you very well. Bit not only that. If u take my advice in the video. The stronger you get the more rewards you can get. But rlly I have so much gems because I don't need to spend them I have all I need. But try to get stronger, which I showed in the video and you'll get there. Do daily and weekly quest as well. Very helpful, speed raid as well.
U mean just roblox account? Not including the password? Cause even if u did get his password he might have 2 step verification and also when the password is entered it notifies the gmail of that account on where ur exact location is when you tried to log in so uhh i might delete this reply sooner or later