Unmissable (well, sort of...)
Just after you get the green pen that lets you draw branches in the second world, you will come to two henchmen who throw bombs. You can kill one by raising a pillar to create a barrier and a vine to create a ramp. The bomb will roll to him and kill him. Once he is dead, wait until a bomb rolls to the second pillar spot, then raise the pillar to bring the bomb to the original thrower. When it blows up and he dies, the achievement will unlock.