Then try spamming them and see what works against you?  I very rarely see StuGs in 1v1 TBH...

StuG G is almost ineffective against infantry. 

Also, its main gun is fixed. 

With just two Chaffees, the StuG G will be sitting there doing nothing. 

Improve your skills.

I use stug's in 1vs1 and the pintle mg and inbuilt mg are actually quite good at damaging infantry, if you manager to get 2 or 3 stugs together you can shred squads approaching you.

But I think this is fair if you paid for upgrade and it's only head on and since it's not turreted I think it's balanced ok