This really isn't too much of a push over. You can pretty much coast through the game using your assault rifle and upgraded Stingray.

This will put you to the test. Keep with a good assault rifle and an MLRA series missile or Stingray rocket and you're in great shape. By the time you finish this, you should have close to or over 1000 health. You're going to need all of it! In indoor areas, use the P79, P80, or P89 bound gun whenever you have an indoor mission.

Keep gaining health, and use the most effective weapons for every stage. Co-op is definitely the way to go, and different enemies are going to require different guns.

Walking Arms should be handled with the Stingray, or go under it and use a good burst assault rifle. Dino mech is going to be some trouble. Try to use the helicopter, get behind him, and spray your assault rifle on to his skin. It is possible to land on his back after a rampage, just jump out of the copter, land on his shoulder, and pump his head full of lead.

There really isn't advice for this, but just make sure you're being smart in your maneuvers and have a co-op partner.

Ditto the Hardest. This is impossible without a co-op partner, and by now, you should have almost all of the great weapons in your inventory. Upwards of 3000 health is a requisite, but surviving more than the first stages is miraculous. But when you're done, enjoy your genocide gun! It deals 1,000,000 damage!

One thing I failed to mention is that each difficulty must be beat on it's own, so keep track of your completion medals. Level 7 is the best level to grab weapons and health, you can usually get about 200 health in an hour if you're vigilant.

There are 171 weapons to the game. The whole weapon list is here

Go all the way to the bottom. It is in table format, so make sure your browser can display this.