(THIS GUIDE WILL HELP ON ALL DIFFICULTIES)
If you are having trouble beating any levels this is what you will need to do.
Note:this game will take some time to get all of the achievements in, I reached 80 hours play time
1.HEALTH and WEAPONS
When you are playing the game on any difficulty when you kill enemies most of the time they will drop items (HEALTH,ARMOR,WEAPON,and MEGA HEALTH),
every time you pick up Armor once and if you beat that level all of the armor you picked up get's added to the maximum amount of Health you can have. Each Armor pick up is 1 or 2 health points.
The easiest level to get the largest amount of Weapons and Armor is the level titled "Assault". Every Walking robot drops 3 items when you kill them, So just do that level a couple of times to get Weapons and Armor which will Help out Majorly in harder levels.
You can probably beat easy,normal, and hard mode if you are lucky by your self, once you get to hardest and Inferno its time for some help.
Now whats you have good amount of maximum health and good weapons you can try some harder levels in co-op mode. Once in co-op mode I would suggest you come up with a strategy and not just run out and die. For example, player 1 will take out the enemies while player 2 takes out the holes and/or dropships. Its all about how you want to play it though.
3. HAVE FUN
If you are tired of playing the level "Assault" over and over again to get armor and weapons or just can't beat a level, then take a break. You don't need to get all stressed out about it. Just play in burst on time.
As Dadmix has pointed out, there are 53 levels in total in this game.