Training
Sniper:
Well, obvously, you have to be able to hit a person from at least 30 feet. So to practice that just find something human-sized and if you make your own target from paper and put it on a door (like I did) try to get a photo of some one you hate, cut out the head and put it on your target. Now, shoot at it from 30 feet away to practice your skills.
Trooper:
A trooper needs to be mobile, in a way that they can move quickly. They also need to be able to use almost any weapon with ease. To train, you could do some jogging, running. Also, spend as much time as possible using your primary weapon so that you can work around the flaws of that weapon (if any).
Mercenary:
A mercenary needs to be quick, agile and intelligent. They need to know when to strike. For mobility, practice like you are a trooper, do jogging and running. Also you need a slight bit of insanity. So, hit your head on the wall for 5 minutes everyday for a week and never doubt yourself even if your plan (if any) is impossible.
Spy:
You need to be quick, agile, intelligent and being vertically and horizontally impaired helps too (being small). You need to know what's important and what isn't. You don't need very many battle skills, but they are a plus.
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