Atomicrops

There are 8 tree monsters in total, which must be killed in one run. Each zone has one golden pupa that spawns when you clear a random camp. The golden pupa then flies to a random spot on that map and will spawn a tree monster. There will be an indicator on the edge of the screen if you happen to lose track of it.

I found it easiest to clear each biome in this order: western, eastern, northern, southern. Then the second half of the map(will need lots of bridge repair kits, or the boots that let you fly) in the same order, so tier 2 biome of west, east, north then south.

Once you learn the flow of the game, this will be a fairly easy achievement. Just set to year one and rush each tree monster with the best weapons you can get.