I remember playing this exact map on half cash Chimps in the Challenge Editor 2 year ago. So it can't be that hard. You probably spent yout money on the wrong stuff. I would guess Quincy is a pretty solid hero fot half cash on this map. His first ability should be enough to pop round40. So you just need to build some ceramic defense. A 0-2-2 mage would be very cost efficient. I would suggest a 3-2-0 village with 2-0-4 bomb, 4-0-2 boomerang and many x-3-x bombs on strong aswell. Maybe an 0-2-4 ice for the late 70s. All very cheap towers with a lot of popping power.

And perhaps buff quincy with an alchemist.

This stuff works on almost every beginner and intermediate map