How to capture a npc lvl 8 AGE2
1: Have an attack hero with atleast 160 attack
2: Have at least 300k warriors that you can dispose of. Wars are easily built, and you do not have to waste archers.
3: First attack send 100k wariors with 160 attack hero.
4: After First attack, send waves of 20k warriors (any hero will do), make sure to send the waves all within six minutes of the first attack. Keep doing this until you finally conquer it.
This has seem to work for me, I have conquered two lvl 8 NPC's using this strategy.
I am right now working on a lvl 10 NPC, I should have a Guide up for that in the next week or so.
Good Luck