In Open maps the geshtutz is just crazy but....there's some things to keep in mind...
At long range, their penetration is crap meaning if you keep frontal armour and distance between than and the T34/SU, you might just win
Also , the T34s/M18s flanking and then siting instead of circle strafing?? wtf?
T70 has M8 armour meaning both the geshutz and marder are likely to miss it and then it can circle strafe them