HOW TO GET HIGHSCORE:
1) Put Buzzsaws in a horizontal line across the center of the screen. Don't let them touch the edges!
2) As soon as you can afford it, spam Proxy mines all over the ground. This will let you blow up all the turtles on-screen.
3) Put 4 or 5 Sentinels above where you placed the Buzzsaws.
4) Put 2 Turrets on each far side of the ceiling. These will direct the shells into hitting the Buzzsaws and staying above ground. You can buy extra Turrets to make sure that the shells stay in the upper half of the screen, however, this is not the most important step.
(note: You can do steps 3 and 4 in reverse order, it doesn't really matter.)
5) Cover the ground with Launchers, so they don't get stuck on the ground. This can get quite expensive after a while, so try to leave gaps smaller than shells between launchers.
6) Leave the game on for 3 hours.
7) Congratulations, you now have one of the best spots in the leaderboard!
thanks!