NPC attacks with Ballista 

 Level One: 50 Ballista
Level Two: 150-200 ballista
Level Three: 250-300 ballista
Level Four : 350-400 ballista
Level Five: 400-500 ballista
Level Six + : not recommended, you WILL have losses

NPC attacks with warriors


Level One: 1200 warriors
Level Two: 3000 warriors
Level Three: 15000 warriors
Level Four : 22000 warriors
Level Five: 32000 warriors
Level Six: 38000 warriors
Level Seven: 60000 warriors
Level Eight: 85000 warriors
Level Nine: 98000 warriors 
Level Ten: 110000+ 

Bellow are charts of both ballista needed for npc farming .

                                                              Min Ballista (Based on Arch & Mil lvls belowl)*

NPC
HBR*
Arch*

4
6

5
7

6
8

8
9

9
10

Losses

Hero
lvl/Attack

1
2
3
4
5
6
7
8
9
10

50
100
170

12
100
140
350

12
50
135
275
550

10
25
125
225
500

10
25
125
200
425
3000

0
0
0
0
0
275

10/40
20/40
30/70
30/70
35/75
45/100