So, arty 15 mission. How ?

eekeeboo: Get a hard hitting lower tier arti that has head room of +2 above it. I suggest the m44, m12, s51 etc maybe the hummel, lorr etc even the german tier 7-8.    You will want to focus on doing the following in game to maximise xp in your arti:   Shooting unspotted targets; if you think a tank hasn't moved but no longer spotted? Send a present to check.    Shooting Higher tier vehicles, as has been said, this gives more xp, especially if you shoot them unspotted then get assisted damage on them.