You're asking the wrong question.
You'd like to be able to play The Trooper? You are able to play at least parts of it? Ok great! So keep practicing. You'll be able to do it better and better. Regardless of that, definitely practice also other tunes.
Learning a musical instrument is not some sort of quest through different songs / techniques that are to be vanquished. Rather, it's an ongoing process where you get ever better at everything. At some point you may have “mastered” individual songs, but that shouldn't really be what you have in mind while practicing.
In particular with a song like The Trooper, you'll probably never be able to play it quite like Steve Harris – but that's fine, hardly anybody can do that. You may however already be able to play it in a way that's good enough for giving the band a satisfying sonic fundament to achieve a fine overall interpretation. Start from there, keep improving.
When actually playing in a band, the really important thing is to not focus on trying to make any individual aspect perfect, but instead keeping the whole thing balanced and round. In particular for this song, it's crucial that you keep the pace and power throughout the performance (in fact, I think Maiden speed it up quite a bit), but not that you actually execute every single note as in the original. That's where it comes in handy to have practiced not only a single technique: there's a lot of ways you can “cheat” in this song, and still get a good result, whereas cramping on to a tab will probably result in everything getting quite poor and feeble towards the end. Even Harris himself doesn't actually gallop all the time – he occasionally leaves out some of the in-between sixteenths. But the notes he does play, he absolutely hammers home.
You can get a lot more creative than just omitting notes. One very effective energy-saver on bass is raking technique: instead of using two individual finger strokes, use the same finger twice on different strings. In a key like E, this can readily be implemented by using octaves.
Related, specifically in a riff like the intro, make use of pull-offs. I'm actually not sure if Harris plucks all the notes, but I would not. Pull-offs essentially distribute the plucking work over both hands. But even if you simplify it to a simple fourths-motif
X:1
L:1/8
M:C
K:em
%%score B
V:B clef=bass
% 1
[V:B] E,E, E,B,, D,D, D,A,, | C,C, C,G,, D,D,
that'll work if the guitar doubles with the full thing.
The only thing that you really can't afford, as a heavy bass player, is to drop out or drag.
— That's of course not to say it should be your ambition to cheat yourself through everything. Your ambition should be to play the song as best as possible, but the way to get there is neither to make it your big goal to play that one song note-for-note, nor to shy away from the challenge completely. The way to get there is to not fixate on one “way”, but instead work in a broad manner to improve yourself. I'd suggest approaching it from three directions simultaneously:
- Slow and meticulous: don't hurry with this, but do try to play each part of the song in a note-perfect manner – at super-slow-motion tempo! This is where you can really take apart the technique, observe what your fingers are doing (avoid double-use of a finger), and of course you can scale up the tempo more and more and approach the real deal in this sense.
- Fast and bold: as I said, note-perfection isn't really what matters for the song. It's no good if you can manage to play the song perfectly, but only 80% tempo and it falls apart completely at anything faster. That's where “cheating” comes in: it allows you to not only push your limit (which can become very tedious and demotivating) but also leap over your limit. It'll fail a couple of times, don't worry about this.
- Playing entirely different songs. Remember, it's no use to entrench yourself in kind of a battle against a single song. Mixing it up with other material will not only broaden your capabilities, it will also help get to that song easier! Even if your only goal as a bassist were to play the song The Trooper in an Iron Maided cover band, practicing only that song would not be the way to perfect it.
