Same as Dang3R i havent touched the game since i wrote the guide, but i can say that the achievement is excluded from the story in a seperate game mode.
I think what you mean is the fact that while progressing through the story you add more types of bombs to the gamemode so it gets "harder". I chose to put it at the end because you'll have the practice from the story mode and know what to do instead of going in blindly.
None the less i hope the guide was still helpful.
*you might want to google "constructive criticism" :)