This grepolis guide comes from our friends at grepoguides.com
Don't rely on resource production of your cities, do excessive farming instead. this is how most resources should come from in Grepolis.
Each farming village can ultimately give you 6000 in resources each time. The mood needs to be over 75% for your looting attack (or 60% if you have researched diplomacy). You need to increase the strength to 100% before the looting attack.
By default strength is 50% and if it's not it always tends to 50%. Strength can only increase by up to 15% each time no matter how many troops you send, and each single unit that takes 1 farm space increase the strength by 0,2%. So for 1% you need 5 single units. When I say single I mean swordman, slinger, archer or hoplite; the rest aren't single.
Example: Let's say you only got swordmen, slingers and archers and the strength of a farming village is 50%. you must send some fast units to increase strength at first. send 3 increase strength attacks of 75 slingers each and a 4th one with 25 slingers. that will increase strength to 100%. at the same time send 250 archers for the looting attack to get the max resources possible. alternatively you can send 375 swordmen. don't use hoplites to farm with.
if you have booty researched you only need to send 200 archers for the looting attack (or 300 swordmen).
Example: Let's say you only got swordmen, slingers and archers and the strength of a farming village is 50%. you must send some fast units to increase strength at first. send 3 increase strength attacks of 75 slingers each and a 4th one with 25 slingers. that will increase strength to 100%. at the same time send 250 archers for the looting attack to get the max resources possible. alternatively you can send 375 swordmen. don't use hoplites to farm with.
if you have booty researched you only need to send 200 archers for the looting attack (or 300 swordmen).