I've played this scenario far too much. Like you, I quickly found that holding Manhay and Grandmenil to be almost impossible after the Allied reinforcements get into play, as their artillery firepower quickly becomes overwhelming. I've been through many different plans since then, most of which have been upset by the damn sneaky AI. There's eventually too many of them, they've too much armour and artillery, and the main objectives we're supposed to hold are deathtraps. I've tried going to the exit straight away, but that requires considerable force as there's usually a lot of allied hanging around it. Getting bogged down in a firefight that far from support is never going to end well. One problem that hit me a lot in my earlier games was that leaving *any* gap to into your rear areas means that you'll quickly be infested. I've had them come from the north, down through the southern gap in the woods and, in one particularly "oh sh*t" moment, straight through the woods. Once they're into the rear then I find that's pretty much game over.
I've started going for disruption, which involves holding the road objectives to the north and north west. Unfortunately, getting to the objectives in time generally means moving fast, which tends to fatigue the troops rather badly, which means they're easy to crush later on. I think the only way to get better than a draw in this scenario is to prevent the Allies from getting organised and from moving freely.
I'm about to have another go at it. I'm going to see if I can hold the three northern road objectives in force and have a couple of companies hold the deathtrap towns on their own. I'll station a battalion in the woods south of Manhay to provide a reserve and to prevent infiltration of our rear. The southern access point will be held by the flak and AT companies. When the rest of my armour turn up I'm going to send them west through the woods at La Fosse, with the intention of them getting to the exit and crushing anything they find there. If I have time I'll swing them back up the road to the westerly road objective, which should hopefully give me a way to get some infantry to the exit point.