As soon at 1-1 starts, head left and attach the two statues. The head will give you a better attack. Keep looking everywhere for those statues, as when you break them sometimes, a coin will come out with a B on it. These are the coins you are looking for. Search very well before finishing each area. You should be able to get two in 1-1, one in 1-2 and two more in 1-3 for the achievement. The boss is pretty tough so you might want to avoid having to fight him.
If you do get to the boss and beat him, you will need to spend coins to move to the next level, but they still count toward the achievement, so collect one or two more that you need in level two.