It applies to the optimal archetypal armies used, there's in fact maybe a dozen of them. For example:
Wood Elves: Legolas, Saruman and Wood Elves
Wood Elves: Legolas, Wood Elves & Grey Company
High Elves: Saruman, Twins and High Elves
Mordor: Spider Queen, Shadow Lord, Morannons & Orc Spearmen
Isengard: Saruman, Shadow Lord, Uruk Crossbows & Orcs
Harad: Shadow Lord, Golden King, Mass Harad & Corsairs
Gondor: Saruman, Gondor & Grey Company
Arnor: Arnor, Malbeth, Twins, Glorfindel
Moria: Spider Queen, Shadow Lord, Prowlers
Moria: Shade, Shadow Lord, Drum, Blackshields
Rohan: Saruman or Legolas, all infantry, Woses allies
Rohan: Saruman or Legolas, all infantry, Grey Company allies
And a few others...
These are the optimal archetypes, once you work out the standard builds then any changes you make are just details in fine-tuning, otherwise everybody uses almost the same armylists from the chosen factions because those are just the most optimal mathematically and give the greatest tactical edges.
Standard build = take the right heroes. Take the maximum numbers you can get. Max out on bows. Know what the army is supposed to do.
Then there's the current meta analysis - you look at the past tournaments and work out which of the best armies have been the most often used and make a counter army for that, example:
- In the 2010 GT there were 6 Wood Elf Armies in top 10. The most effective counter-army to Wood Elves is Harad, because it has the ability to outshoot Wood Elves while with Shadow Lord and it has D4 all around which is optimal against them. That and the addition of Corsairs and their throwing weapons makes Harad/Sair army the best counter-army to Wood Elves.
If you expect the meta to stay the same because of scenarios (it did stay the same) then you're best off taking the best counterarmy for the meta.
You don't *just win* a GT... it's a few years of hard work to get all the experience and relevant material for a properly build list... then there's the playing skill, which often is very similar to other players and you only can beat them by playing as good as you can and exploiting every mistake they make - and everybody makes mistakes, that's why the game isn't luck dependent - if everyone played perfectly then it would be based completely on luck. Essentially the skill is in not giving up your army edge and in exploiting enemy mistakes