I was using Soviet Line to get this. My commander/crew was rank 8 shortly after I got my Tier 9 (IX) heavy tank fully upgraded. I started with this commander at tier 5 (V) I think so not as early as I could have. I always paid the silver to transfer them to a new tank and train them @ 75% starting new tanks (usually cost 100,000 -110,000 silver for each transfer).
Once she was Rank 8, had 8 skills with her also and transferred her to my only Tier 1 (I) tank BTS-1 I think. I fully upgraded my tank and equipped it with 3 equipments all 25,000 silver or so per upgrade. Played about 10 games before I got it. Easier done in Tier 1 (I) matches as your enemies take less hits to kill. Just remember to get the final hit in. Province and Liberty Falls are best maps for this I think, I earned it in Liberty Falls. Russian line is definitely not the easiest to do this with you but I was stuck with my highest commander. Remember to get at least 3 kills (I got 5), and survive. Find a cover spot with a great overlook n if you are like me in North American servers and try to play early in the morning and you'll fight mostly bots. You can tell they are bots by pressing select, and trying to view their gamer card, (you cant). Usually thier names are Castor, Wei, ect.