If you want to get this particular achievement as quickly as possible, simply play the whole campaign in Easy. However,I recommend playing in at least Hardened in order to get the achievement for completing each individual level (you won't get them otherwise).
There's nothing wrong with playing in an easy mode first though - If you do decide to play in Easy or Normal, you will at least gain progress to upgrading your weapons and gear, which can help make subsequent play throughs on harder difficulties much easier.