NPC farming

1. BUILDING NPC CITY
If you want your own NPC city you can build it by this way - conquer valley of level X ... build your new city in that spot ... go to another city you have, click main building - cities- abandon your new created city... now the city of level X will be ready for farming / conquering.

2. RESOURCES/TROOPS/DEFENSE
NPC city resources are replenish each 8 hrs, A barb city will take approx 1 hour to regenerate all to full (10% of the full amount per 6 minute tick). A barb city will also immediately regenerate approx 18% of all lost wall units each time some are destroyed. Example: if you destroyed 1000 towers and hit it again 1 second later 180 towers will be back.

3. ATTACKING
Attacking low lvl NPC cities are mainly for resource and gold, this is done with ballistas to achieve 0 loses. The maximum level without loses is level 5, level 6 or more have trebuchets so you will always suffer losses there.
But there are ARCHER TOWERS... so you need specific tech level of archery and Horseback Riding (HBR).

Archery 08 / HBR 03 (Massive losses due to being in tower range)
Archery 08 / HBR 04 (Smalls losses from units but @ tower safe range)
Archery 08 / HBR 05 (0 Losses )
Archery 08 / HBR 06 (0 Losses )
Archery 08 / HBR 07 ( Small losses )
Archery 08 / HBR 08 (Massive losses due to being in tower range)
Archery 08 / HBR 09 (Massive losses due to being in tower range.)

Archery 09 / HBR 01 (Small losses from units but @ tower safe range)
Archery 09 / HBR 02 (0 Losses when using over 575 ballistas )
Archery 09 / HBR 03 (0 Losses )
Archery 09 / HBR 05 (Small losses from units but @ tower safe range)
Archery 09 / HBR 06 (Small losses from units but @ tower safe range)
Archery 09 / HBR 07 (Small losses from units @ tower safe range)
Archery 09 / HBR 08 (0 Losses )
Archery 09 / HBR 09 (0 Losses )



Archery 10 / HBR 0 (0 Losses )
Archery 10 / HBR 05 (0 Losses )
Archery 10 / HBR 06 (0 Losses )
Archery 10 / HBR 07 (0 Losses )
Archery 10 / HBR 08 (0 Losses )
Archery 10 / HBR 09 (0 Losses )
Archery 10 / HBR 10 (0 Losses )

(copied from syoganai1111 thanks)

4. NUMBER OF BALISTAS TO ATTACK
This number depends on your hero ATTACK attribute, the safe numbers for heroes with no attack with 0 loses are:
(Note: You still need one of the above zero loss Archery/HBR combinations)

20 Ballistas vs Lvl1 NPC
50 Ballistas vs Lvl2 NPC
170 Ballistas vs Lvl3 NPC
250 Ballistas vs Lvl4 NPC
400 Ballistas vs Lvl5 NPC 420 Transporters

DONT FORGET TO HAVE RIGHT AMOUNT OF TRANSPORTERS!! they are similar to ballistas numbers so be sure that if you want farm you will take everything.

I have found some of these numbers to be wrong in my npc attacks.
For lvl 5 npc you are safe with arch 8 hbr 5 sending 500 ballis. When you upgrade to arch 9 or 10 you will have losses until you reach hbr 9. For lvl 3 npc you only need 120 ballis and 220 for lvl 4 npcs.
For the number of transports adjust to your logistics level. The easy way is to simply send enough to take 100% of thier resources. The amount needed for that is as follows.
NPC Resources
1 215,000
2 355,000
3 1,200,000
4 2,260,000
5 3,990,000
6 5,200,000
7 6,800,000
8 11,400,000
9 16,850,000
10 22,300,000

If you want to farm anything past lvl 6 wait until you have arch 10, hbr 9 and most other research at 8+ to minimize losses.

Honestly, for level6+ you're best off using either archers+rainbow (move into range faster, so take less damage before firing), or catapults+transporters. Catapults should be able to take level 6 and 7 NPCs with no losses, but I'm don't know how many it'll take.

Hope that helps a little.