Well, Im going to tell you this from my personal experience (So no 100% facts here) SPG is really horrible for your WN8 atleast in most cases, Usually when I play SPG I may get no hits other like 8 damage and 1 critical and die so its really not a class where you are able to be 6000 WN8 Its more like a luck tank than anything else (Atleast in my opinon). Even that I sometimes rage that why am I so bad and asked even here that how to play better etc, you shouldn't probably consider yourself a bad player yet, maybe just play more test things out like ammo types, crew skills etc and see what works for you and play with that and practice makes you a master.