In town, there are several orange trees (and a couple large green ones). Attacking these trees will cause them to drop coins, which you can collect for progress toward this achievement. Upon collecting coins from trees 100 times, this trophy will unlock.
Note that every tree will drop gold but not always on the very first hit. Testing this theory, I hit one tree 28 times before it dropped. Once you've collected coins from a tree, you need to leave the area and return to repopulate it with gold.
While grinding this out, I'd hit each tree up to about 10 times before moving to the next. Overall this is faster than only hitting each tree once before reloading the town.