The easiest level (for me) was the Infinite level, Minefield at the end of the Medium levels, because the bad guys come out slowly, and you can build a winding path to the red destination. When you get enough $$ to upgrade one vortex tower do so until you get to Level 7.
Cheevo pops when the round finishes - and it only finishes when you finally lose all your lives. I got to a point where nothing made it thru to the final goal, so I simply deleted a couple of guns to provide a direct line to the red goal.