If the problem is that the Germans just can't hold, one thing you might want to look at is a house rule that the Allies can only attack on 'minimize losses.'
Yes, I know it's a house rule, but there's nothing actually in the Bible that forbids house rules, and it would be a simple one to implement. It also has the potential of nicely simulating actual Allied behavior in the field in 1944-45. They just weren't prepared to take heavy losses in the attack, and the Germans were able to get a lot of mileage out of that fact. Not to take anything away from the German defenders of Cassino or Caen, but one reason they were able to hold on so long is that after each bloody nose, the Allies would quickly draw back and try to think of what else to do instead: they sometimes suffered heavy losses, but they weren't willing to accept them intentionally.
The house rule also has the advantage of allowing the Allies to do well against German counterattacks -- which they largely did -- and to successfully drive beaten Germans before them -- which they also did. If the scenario is revised to create still more formidable Germans, that may no longer be the case. For example, at some point uber-Germans will be able to simply crush the Anzio beachhead and will indeed successfully break through in the Ardennes.
I am not Charlie Hebdo