This game is not too difficult to complete on normal.
The difficulty cheevos DO stack so when you get this cheevo you will also get the easy difficulty cheevo too.
There are in total 17missions to complete with mission 17 being the final boss battle.
The final boss can be a little tricky but there are some very good guides on the "Sleep when im dead" cheevo page.
Some general tips to keep in mind when playing through the game:
1. Grenades, these are your best friend and your worst enemy. Whenever you see more than 1 person grouped together throw a contact grenade at them by double tapping RB.
2. Abilities. Dont forget to use them. thats why you collect intel so use it, Explosive bullets are wonderfull for dealing with big mini gun dudes, The pulse wave can be invaluable in later levels when you get swamped as it will kill all enemies in a room.
3.Weapons. Once you unlock it, the KALUGA will be your best friend, 100clip magazine and it shreds enemies to pieces.
4.Cover, if your gettin hit and the screen goes red get to cover, regenerate, pop out and carry on.
5.Landmines. these are great, if you get to a point in the game where you have to wait for intel to download, lay landmines where you know the enemy will come from before starting the DL and your life will be much easier.
Anyways good hunting and enjoy.