Mighty League general strategy
For the Mighty League, it's very hard to give a specific strategy to finish the levels that lead to the Master League, because the levels are changing everytime (although I think Rovio has a pool where they choose from, so some levels are returning for other players).
However, after playing the league for a few days (I chose to ignore it for a few weeks because of the obvious money-making game-destroying setup) I found the following things, I call them strategies, useful.
* Try to get as many tickets as possible. You get three free tickets every day, but you can add tickets by watching stupid ads (I especially hate the 30-second one of Gates of Epica). After watching an ad, you got a two minute delay before you can watch another one and after 2 or 3 ads, the ads get blocked. But it's unlocked again after an hour or so and again you can add 2 or 3 tickets. Do this whenever you get the chance, even when you completed todays goals, because you will need them. And personally, I think €2,99 for 5 tickets is ridiculous.
* When you open a level, before you fling any bird, screenshot it. Then head to the walkthroughs here on ABN and find the level that you just opened. Yes, all levels are taken from the "old" ABO levels and all of them can be found. The background (scenery) of the level usually gives you a hint on where to look.
* If you found the level, you can either read the 3-star instructions, watch the video and remember the level.
* If you have a second device with all ABO levels unlocked, use it to practise the level until you're sure that you are able to get the points necessary for the star.
* Now return to your game. Try to fling your best to get the star in one go. One advantage is that you don't need a 3-star score, another one is that you get a time bonus if you're fast enough. Usually those two will greatly help you to get to the star score.
* If you missed the star target, or if you didn't have a second device to practise the level, exit the league before spoiling another ticket and find the normal level in the game. Then practise, practise, practise until you're sure you won't spoil another ticket.
* As @20qmindreader pointed out in the comments below, it's possible to play the regular levels and (deliberately?) fail to finish it. You are then offered to watch a video to get a powerup. This way, you can replenish your supply of nuclear bomb birds without paying real money. Thanks @20qmindreader!
Working like this has greatly helped me to finish 3 levels a day with an average of 2 tickets per level, up until Silver. I hope it is of help to you too.
As soon as you reach Gold, you will notice that the star score is much higher than you can ever get using only your normal birds and no PU's. I've checked a few against the leaderboards and without exceptions, the star score is higher than the highest score on the leaderboard. So for Gold and Master, you have no other choice than to use PU's. Obviously, this is what Rovio wants (and I don't).
If anyone has other tips or strategies that make reaching the Major League easier, feel free to comment. And of course, power-ups can make life much easier too, but here on ABN we don't use power-ups, do we?