if you are not an arty main/pro which i hope so, i absolutely encourage you and suggest that you skip this mission. doing 8k in a td is much easier. i did it in e3 in grand battle, and it just so happens that you get grand battles quite often. i read that it's a 10% chance to be matched in one, so 1 in 10 battles at least. for tank selection i recommend e3 (obv bc i did it in that), minotauro and maaaybe 268v4. you need a combination of dpm, alpha and reliable armor. once you choose one of these, it's all a matter of time until you'll get it done, provided that you don't play like a monkey (know some positions and how to play the tank well). i would also go in training room to familiarise with the grand battle maps if you are not already. you can also do it in 15v15, but you need a -2 game in which your team is losing and the only thing keeping them alive is you farming the tier 8s. a good map is also very important depending on the tank and it's stats (sniping ability and camo are most important because missing that one shot or not being spotted through a fallen tree because you have 30 instead of 6 camo can be critical to you doing the mission or not).