Once both cities are liberated, secure the area by using 
armor to track down the few artillery pieces, then push 
both battle groups north toward their objectives 
(Viipuri/Tali and Sortevala), leaving only a small 
garrison behind. You might want to take the opportunity 
to pick up some new KVs. Some Finnish infantry will slow 
you down - and they are, pound for pound, much better 
than the Russians. They'll put up a tough fight, but 
don't get bogged down: keep moving. Get your engineers 
in on the northern side of Viipuri, and lay in for the 
siege. It will take heavy barrages to loosen these hexes 
up, so don't commit your infantry and armor too soon. 
Chip at them for a couple turns, then go in for the kill. 

As with any good battle plan, the key is to stick to your 
objectives. Don't try to crash the whole line, and don't 
get bogged down in the forests or swamps. Properly 
weakened with artillery fire and polished off with a tough 
armored punch, the Finns can be beaten with a major victory 
in less than ten turns.