I managed to buy almost everything with potatoes. Then I switched to watermelons. Carrots and grapes didn’t really click for me, though maybe I just didn’t find the right approach to them.
Grinding coins was pretty boring, actually — possibly because potatoes turned out to be so convenient that I didn’t feel like trying anything else.
The boss took some running around. Sometimes I couldn’t keep up with reading the attacks — with the circular spikes, you have to guess from the visible part of the circle in front of you, and the circles on the ground are barely noticeable. That said, I don’t think I ever actually got hit by them. The radial spikes, on the other hand, were quite hard to dodge.