Desktop Tower Defense on Hard

I recently spent a few days obsessing over Desktop Tower Defense.  My goal was to beat the game on "Hard" mode without losing a single life, and I hit that goal 4 times.

The frustrating part was that I couldn't design a grid layout that would always win.  There seems to be a certain amount of luck involved.

The biggest hurdle was Level 48 + Level 49.  The reason these levels were so challenging is that the bosses from Level 48 (the Spawn creeps) would still be winding their way around the maze by the time Level 49 started.  You don't really have control over which creeps your towers will prioritize, and so the bosses on Level 49 (the Flying creeps) would often zip through without taking a lot of damage.

If anyone has tips on how I could improve these grids, let me know:

(That last grid might be a little misleading, because I'm pretty sure I was selling off some towers at the end in order to get the Level 50 creeps back towards the center.)