How on earth do I deal with them? I've tried several different tactical approaches and they all just shoot me or sneak up and assassinate me (this is the final part of the 3rd mission I believe, where you are trying to clear the evac zone), and this is only the normal difficulty! Any help is greatly appreciated!

Use a vehicle machine gun or a shotgun....

If this is the part where you first meet them at the rendevous point, you'd be better off using cloak to sneak into some undergrowth and then sneak around until you identify them. You should be able to see them even in cloak if you look carefully.

The first one I'd probably advise beating him to death and taking his rifle. After that, you can use his rifle to double-headshot the rest for easy kills. Tactics are your friend in Crysis 1.

I started my Delta walkthrough and would have thought this section would take me endless amounts of trys to master but nope did it on my second try.

Take them out from a distance, decloack and cloack to make your last position visible to your enemy and then take them out with your sniper rifle.