I FINALLY DID IT!!!
Without cheating too.
I realized what people meant when they said "dont upgrade the blues".
Blues have equal slowing power regardless of their level versus the enemy level. Its much better to have a long row of blues for severe slowing and then a handful of reds for damage.
On Switchback:
I put one red in the big block of 16 square. I let him wreck havok until I got enough for a blue. Then I just kept plopping down level 1 blues until I had a buttload of money and needed more firepower. I added one Red Rocket at every other wave and never was in any danger.
I also found out that Red Rockets actually shoot faster than Little Spammers if the enemy is really close.
For those that dont care enough to figure it out:
Use lots of weak blues and a handful of powerful reds. Greens might seem like a good deal but they are actually very limited in firepower. Never use them.
Reds have the best bang-for-the-buck, especially level 1 Red Rockets.
Purples have the worst bang-for-the-buck considering how slow they fire. Never use them.
Blues have rotten bang for the buck but level 1's are an excellent and cheap way to slow enemies down to a crawl. Mix them with Reds and let them eat the enemy up.
The best bang-for-buck in any unit is always at level 1. Better to add another level 1 unit until you cant add any more. THEN upgrade.
I still only had 620,000 points and a little under a million bucks by level 50. But I got the idea. 33% interest. The power and range bumps are junk compared to having a crapload of money which purchases much more firepower.
http://pics.bbzzdd.com/users/shortylickens/Tower.jpg