Energy runs out quickly in this game if you are playing a difficult stage over and over, losing and trying to beat it, and it takes forever to come back. You can speed that up though by going to the date and time settings for your phone (or other device) and setting the time an hour or so ahead. If you do this and then go back to the app, and then set the time back to normal, and then go back to the app again, your energy will be completely restored no matter how much time you added on.
If you need to get a lot of coins in a short period of time, go to one of the really easy stages, especially level 1, and play it over and over. You’ll earn the same amount of coins as a reward for beating it that you did before, and you won’t lose any energy if you beat the stage.
With most stages, sneaking up the side of the stage with trick shots, trying to drop massive amounts of bubbles at once, is a good idea. However, there are some more difficult stages, such as stage 16, where the best bet is simply to go straight up the middle of the stage, popping groups of bubbles and allowing the bubble stack to lower itself.