I was trying to beat the Final piece on hard and it was getting very fustrating. Then I saw a post by taylorxxx on achiev.org about how he hides ( crouched down buy the unlimited ammo crate in thr rise above level 7) Well guess what, it works like a charm for this level. Just immediatly crouch down behind the unlimmited ammo crate and use the plasma riffle on the dinosaurs as they come around the crate. I did this and it worked with the first try.
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a very small tip for a pretty easy cheeev, but still - Xbox360Achievements.org
This will save you a lot of time and a lot of aggravation. The dinosaurs could still hurt you if your not careful though. Oh, if you don't repair the fortified area you will get that trophy too. Hope this helps. Good Luck!