The achievement requires you to get 30 Small Ants and 10 Large Ants. I have played through the Amazonian Expedition for the Campaign several times and have found out that there is only 45 Small Ants and 9 Large Ants in the Campaign level.
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Therefore, I completed the Amazonian Expedition Level to get that achievement and then went back to Freeplay for the Amazonian Level and killed 1 more Large Ant and the achievement unlocked.