Here's some general advice on learning difficult parts that may help...
1. Invest in a metronome and use it. Until you've been playing for many many years, it's hard to keep an accurate tempo at slow speeds. (Yes, believe it or not, playing slow is in many ways more difficult than playing fast.)
2. Set your metronome at a very slow speed - less than half of the song's real tempo. I like to start at 60 beats per minute.
3. At that slow speed, isolate the biggest problem. In "The Trooper", that is likely to be the right-hand gallop picking rather than the left-hand chord changes. Work on an accurate gallop rhythm with a single, non-changing power chord on the left hand. Do it until you've got it right - and then keep doing it for another ten minutes. Yes, you read right - ten minutes nonstop of playing a gallop with no mistakes. You want to train yourself to do this rhythm so well you could play it in your sleep. It's boring as hell, but it's worth every second.
4. Now you can start increasing the metronome by small amounts. I'd suggest increments of 4 beats per minute - so you're going to 64, then 68, then 72, 76, 80, etc. Play until you get the gallop right, and then add another five minutes of perfect galloping at each tempo. Note that if you try to get up to full speed in a single day, you'll need many hours... so spread this out over a week's practice instead. But after only 15 increments, you should be able to gallop at 120 beats per minute. Not yet full speed for "The Trooper", but fast enough to move on...
5. Once you can gallop at 120, try adding the chord changes on the left hand. If that addition now makes you mess up the gallop rhythm, slow it down again. Try 100, then 80, then 60... find the tempo at which you can play the riff correctly. If you've got the feel of the gallop down with your right hand, it shouldn't take too long to get the left hand correct at some slower tempo.
6. Now you start increasing the tempo again, in 4-bpm increments. At each stage, make sure you can play that riff flawlessly for much longer than necessary. I'd do about 2 minutes of it, but remember that I've got many years of experience... you may need more time. Don't rush to the next tempo! Be absolutely sure you can get it absolutely right at one speed every time before moving up.
7. Eventually, you'll have this riff down at full speed. And due to the prominence of gallop rhythms in metal songs, spending a couple of weeks or more to learn "The Trooper" will pay huge dividends down the road. There are few gallop songs which are easier than "The Trooper"; if you can't nail this song, don't even think about trying some of those Iced Earth riffs. But mastering the gallop on this song means you can transpose that skill to hundreds of other songs, and you'll learn the next one in days instead of weeks ... and eventually, it will be hours instead of days.
I agree the song Running Free would be great for a beginner on a guitar.
