The difficulty achievements DO stack, so I advise you do Admiral difficulty first to avoid having to play through additional times. This game is incredibly easy even on Admiral difficulty, but for those who are not the best at FPS, or are having trouble with the game here are a few pivotal tips to keep in mind:
-Support from your ships can turn a tough battle into an easy victory. Remember that there is a recharge on it, but enemies rarely push up to where you are hiding (Thugs and mines not counted in this) giving you essentially as much time as you need to plan your move. Do not be afraid to use support to clear out tough enemies halting your progress.
-The carbine / shotgun combo is one of the most powerful in the game. The shotgun keeps anything that gets too close at bay with awesome stopping power and the carbine takes foes down from a short-medium range easily. remember if using these to look for ammo boxes often.
--The railgun is a good substitute to a carbine in your plan is it has the medium-long range capabilities, but also when charged fully, has a devastating explosive attack.
USE COVER OFTEN!
Dropping in and out of cover is the single most important aspect to avoid frustration in this game. Cover will protect you from everything but mines and thugs. Cover also means the difference between life and death when it comes to getting pinned down in a firefight and needing time to heal.
One Word of Caution Regarding Admiral:
You will not be able to simply run through this without worrying about enemies. Although the game is easy, enemies will shred you apart when more than one is taking crack shots on you unabated by cover or allies.
You will fly through this game, but be careful about rushing ahead without surveying and being sure you are prepared to move on. Check your ammo and check your ships before moving onto that next sequence and then realizing you should have restocked.