To conquer a level 10 NPC, you will need to ensure you have at least 9 hero's with 100+ attack as well as a level 10 rally spot. Those hero's also need to have loyalty at 100 before attacking. Your archery, military tradition, and iron working should also be at level 10. You will also need ample food supply to maintain large amounts of troops for at least 1-2 hours. Plan to conquer a level 10 NPC that is no more than 5 spaces away from the city you will be sending your attacks from because of the timing of the waves.
Depending on the distance of the NPC, you will need between 225K-300K archers, 10K or more ballista's, 100 warriors, 100 workers, 600 pikemen, 100 swordsmen, 100 cavalry, and 100 cataphracts. It is also very helpful to apply war horn, corselet, and penecilin before attacking. If you have a war ensign, this will be helpful for the spearhead attack (first attack).
1. Reinforce your attack city (from where you will be sending the attacks from) with the troops, hero's, and food needed for the conquer. Ensure all hero's loyalty is at 100, and are upgraded. This is also the time to apply war horns, penecilin, and corselets.
2. Scout the NPC continually until there is a hero there that is level 10 or lower. Then, do not scout again; it will stay at that level throughout the battle as long as you don't send anymore scouts.
3. For the spearhead attack, send 9K ballista's, 1 worker, 1 warrior, 1 pikeman, 1 swordsman, 1 cavalry, 1 cataphract, and 90K archers (sending 1 of each is called a "rainbow" or "meat-shield" and helps get your archers past the various range zones). If you have applied a war ensign, you can increase this amount to reduce overall losses.
4. Each wave following the spearhead should hit the NPC every 3 minutes. Your spearhead will take longer to arrive and return so you will need to watch your timing very closely. For each wave after the spearhead send 30K archers, and 1 worker, 1 warrior, 1 pikeman, 1 swordsman, 1 cavalry, and 1 cataphract. To reduce losses of archers, you can add 1K warriors to each attack if you have them available.
The attacks need to keep hitting continuously every 3 minutes so you'll need to make sure you allow enough waves to ensure they are returning in time to send upcoming waves out in time. As each attack hits, you need to cure troops in the medic camp right away and as they come back, upgrade your hero's before sending them back out.
The lower the loyalty drops, the less losses you will have. Also, keep in mind that the NPC regenerates troops regularly so if you fall behind on timing, make sure you do not go past 5 minutes between the waves. The longer time you wait to send out the waves, the more it will regenerate.
If you do not have the war ensign or level 10 rally spot you can still do this out of a level 9 rally spot but you will have a much higher troop losses. Example, your first wave may crash completely. If that happens, do not stop with the attacks because you most likely took out most or all of the defenses and troops. This has happened to me before but the most important thing is to not stop sending the waves; they must continually hit.
There are other effective methods of taking a level 10 NPC, however this is the one I personally use since it works for me. Feel free to try your own ways and if you find another way that works well for you, I am sure all the rest of the Arturians would be greatful if you were to post it as well.
adock