I never had my elevators touched with this system:
Levels 1 - 10 Buy machine guns and upgrad TWO guns on each elevator (you will be replacing the third with a laser.) It's better to upgrade two machine guns on one elevator than one each on two if you ge really strapped for cash, but try to keep your elevators at the same level. Always buy elevators and upgrade them to your current elevators strength ASAP.
Levels 10-15 Start thinking about getting lasers on your elevators. You are going to need them for the last two levels. Just one on each elevator is fine and don't upgrade them until the last two levels.
For level 15 I have a level 2 laser on each elevator, two elevators with two level 5 guns and two with a level 4 and a level 5 gun. The monsters never even got close to me. Just park your "big" elevators on the levels with the big monsters and watch the carnage.
A few tips to get the hard badge:
1. You're not going to be using your bullet time as much for dodging as you will be to position your cannon to start a strafe. As soon as those purple guys popping out, bullet time for a moment to line up a strafe then strafe in real time. They never lead their shots so commit to the strafe and you'll be ok.
2. Never bullet time for the oranges. I don't care how many are on screen. Learn their shot timing and adjust. You shouldn't have to bullet time for the tower either unless there's a ton of them.
3. Stay out of the corners. That's where they get you. Try to stay away from the edges in general.
I love this game! It's a little on the easy side (especially with the 300 diamond gun which can be aquired without donating), but it is insanely fun. I laugh at the people complaining about microtransactions. Microtransactions are how a lot of game makers (flash, MMORPGs and browser games mostly) make a profit that allows them to continue making games. Get over it.