Well, first obviously run your infantry away as soon as you catch sight of one. If you stick your panzershrek-equipped Panzergrendiers around to try and fire a volley off, then you were asking for it when they get fried. The key is to run WELL BEFORE it gets close enough to burn you. Otherwise it will chase you down and kill you on a retreat.
Much like trying ZiS's anti-tank guns vs flame Halftracks, Paks will also likely be circle strafed and fried unless their initial placing happened to be ideal.
That leaves tanks. The KV-8 has a peashooter for a main cannon (equivalent to a T-70). So a Panzer IV would have no trouble taking one out.
However, nothing is in a vacuum. If they have an SU-85, then you will need to get creative. You should at least have 2 or more Panzer IVs in this case. Ignore the KV-8 and go for the SU-85. Use smoke, flanking, blitzkrieg, etc. Really, it is a crapshoot that depends entirely on the situation.
Just keep your infantry away until the threat is dealt with.
And you won't be fighting Guards if they went for KV-8, by the way.
And it's damage should be considerably better than a FHT. It comes much later, cost much more, and is doctrine specific (basically the only point of choosing the two respective doctrines).
The problem is, thtat you have to keep your tanks along with your infantry when they retreat, because otherwise the KV-8 just toasts them in your base...
As for the guards, they are only a problem in 2v2 and upwards.
Btw I did't see that there already is a KV-8 thread so this one may be closed or merged.