I don't know exactly what it is. Comparable tanks (guns) are the comet and the black prince it is 'easy' to score 900-1000 average on those. For me the AT15A is very difficult to keep above my average overall exp (thats not good if you think of all the no-premium battles and 'noobphase'). The only thing that can save this tank is a big creditboost or something, since it is ruining stats xD
Ah right I can see where you're coming from now. The thing is this is a TD so it has to be played different to the comet or black prince. Well maybe not too different but your play style has to change to allow for its slow speed and lack of cannon. For example if you get tracked in this you've no turret to spin in order for you to mount some sort of defence. And I'm not trying to teach you to suck eggs. Of course you'll already know all the above. I still think it comes down to being able to forward plan and read situations. Sadly my patience lets me down and I make stupid decisions because of it. It's credit earning isn't massive though. On my mastery badge I only got 44999 credits (non premium) for 3448 damage. Not great.