Warrior farming should be used on Lv7,Lv8 and lv9 NPC's. You will also need to have regular warrior drafts from Historic cities. Level 7, 8 and 9 NPC's have much higher chance of blue gems dropping, so you can hit these in conjunction with you ballista runs.
On LV 9 NPC's ratio is about 6 blue to 4 yellow
If you have all LV100 heroes
Set all gear to Intel. You need to send 3 waves of 100,000 warriors at the NPC on after another. Next you camp another wave of 100,000 warriors to hit every 4 minutes so 4,8,12,16 minutes etc. The 3 hits at the start all have a good drop chance because you kill a few troops, every 4 minutes the NPC will regenerate enough troops for the drop rate to go up high enough again. Its possible to acquire 9 blue gems from 9 hits some times, and gems seem to drop about 60% of the time.
If you have Hero's at different Levels
If your heroes are at different levels These runs can be used to level up your heroes when they get above lv30. You still use the same method as stated above, triple hit then hit every 4 minutes after. Send your strongest hero on the first wave, then send the next strongest/next highest level, then keep repeating so the lowest level hero will be in the last wave.
Try to have uprise full if hitting with low heroes.
The difference with hitting level 7,8, 9s is you get a lot more blues than orange gems compared to lv5 NPC's and lower, therefore you can start using them at lower levels if you want.