I thaught this achievement was impossible until i saw this video:
This guide was a bible for me.
Some important aspects i had in mind.
It's fundamental that you save your game continuously. You can't lose a keshik with double attack, so if the AI kill you one of them, you will have to reload.
Every time you defeat a civilization, you will gain a free tech. You need to calculate the time when this occurs so you can unlock a military tech that demands a lot of turns.
Every time you will start a war remember to apply the share resourses for gold trick.
I started with the Jin Dinasty and with 82 turns left I declare the war to China.
72 turns left approx. I sent a group of 6 keshiks to Persia. The other group will defeat China.
65 turns left approx. conquer Korea and prepare the war againts Japan.
58 turns left approx. the group of 4 keshiks that came from China started the war againts India.
58 turns left approx. this point is very important. Againts Persia I captured the first city I found traveling across the road and them I captured his capital. Them I firmed peace for two of his cities. You couldn't lose time because the others civilizations will upgrade their defenses and units.
54 turns left approx. I started the war againts Arabia.
39 turns left approx. I started the war againts Byzantium.
36 turns left approx. I conquered India. This group of keshiks will finish with Persia and them they will travel to north west to Russia.
24 turns left approx. I started the war againts Russia.
It's very hard but after study this video you will have a great opportunity to get it.
Credits to CaptainCermaine for the big help.