How would you defeat this stack?

If that stack is attacking, a stack of tank destroyers lvl 4 in a forested province will deal with it nicely. Or at least cut it down a lot.

If that is a defensive stack then a counter stack of RRG's should work.

But what do I know...

You have to have railroad guns or ranged units of your own and you have to rush the enemy. Tank destroyers are fairly effective at attacking heavy tanks if you've researched them enough.

TD are good against Heavy Armor which a RR Gun are a part of they are stealth in forest and are more effective on defence

Put a stack of lvl4 Militia in his path to lock it down, then roll in with Artillery.

or

Attack Bombers.