Don't use a Pak, KV-8 eats Pak for breakfast. Your best bet is to hope you have a tank nearby, you won't be able to kill it in a timely manner, but you might be able to body-block it long enough for your Infantry to get away.
I'm not saying you'll kill it, but he'll reverse to get repaired as long as 2-4 shots penetrate from either Pak or StuG. At that point, you know he's got the KV-8 and you should get some extra StuGs or paks.
What's happening here? Just have a pak and a stug to cover the retreat of your infantry.
Don't faust it...just retreat right away and let your pak/stug do the work.
Why does everyone cry OP anytime a unit kills one of their squads?