All credit for this goes to DXS on YouTube.
Above is a full guide on 3-staring all levels in the game. A lot of what I've seen so far shows that the problem is not dealing enough damage, but rather when/where to place towers.
DXS beats all levels with mainly upgraded arrow towers, with barracks backing them up and other miscellaneous towers mixed in on occasion.
Please note, this guide is not bulletproof. There are levels where I've followed his instructions to the letter and still lost, but that's due to tiny amounts of RNG with the enemies. Waves are not *exactly* the same every time.
You may also want to innovate once you've got a good cash flow later on in levels. Several times I've replaced arrow towers with barracks against what DXS is doing in the video because troops seem to get by despite the video.
Good luck, have fun!