I'm about to start on Impossible, and I was hoping that you guys could help me out.

what weapons are good to stick to and upgrade on this difficulty ?
is it that much harder ?

any other tips would be appreciated.

Tier 3 Engineer here I come.

I am just going to stick with the Plasma Cutter. That way you can fully upgrade it - and you'll get your "one gun" trophy/achievement.

Conserve ammo, make sure you always buy ammo and health packs to carry around, upgrade the suits HP and weapons damage first, don't get hit by enemies ;)

Like sammy said, Plasma Cutter only, upgrade weapon damage and HP first and conserve ammo. Save lots and reload if you think you can do better on ammo.

If you want you can buy a Force Gun or Flamethrower to take care of those little things. They can be quite annoying sometimes. Just don't upgrade them.

ugh, just the thought of using the flamer, it was soooo hard to get the 30 kills for the achievement.

that's one bad weapon.

I like the Force Gun tho, I'm already close to getting the one gun achievement.

I'm sticking to the plasma cutter and the "machine gun" (i forgot how it's called, but you probably know which one I'm talking about). Both of them are easily upgraded to the maximum and are very strong. And carrying only two weapons means lots of bullets for them.