This level was one of the easier ones for me, so I’m not sure why everyone hates it so much... I will try to tell you what I did.
Three stages of this last level as I see it:
Sniping the Mortars
Defending the hill (halftracks and infantry)
Destroying the Tigers (with infantry!)
Hill 400 - Defending the top of the Hill
Sniping the mortars: (suggest: sniper rifle and Thompson)
Start in any bunker, I chose northern one because it was close to the main bunker, and slowly worked my way around the outside. "Oh, how enlightening!" you say. This seems like really obvious advice, but move slowly because all of your (really helpful) AI players follow you around, providing plenty of covering fire for the most part! I don't suggest using smoke grenades on this part as it can hamper your allies who are doing a rather good job at ganging up on any enemies trying to sneak up on you.
As I was going around, I did not worry about the mortars trying to hit me, as they really don't, as far as I can tell. Go for the closest one, not the one on the other side of the hill.
Be aware that some of the mortars are single guys, others are two. You have to take out both if there are two (when the star disappears, you have taken it out) (yea, another obvious comment, but I thought I would throw it out there).
Defending the hill (halftracks and infantry) (suggested: Thompson and panzerschreck)
For this part, it gives you a chance to reload on ammo etc etc etc, and for this I head to the 3 machine guns near the main bunker. I find the west-facing one the most useful (faces the side that the halftracks come up on). I find that my troops did quite a good job at defending my back when using this turret, allowing me to spray anyone coming from that side, and duck down when getting hurt. If the mounted gun is not working for you, just hide behind the crates it is mounted on and pop up using a Thompson.
When the halftracks start coming up the hill, you can try (if you want) to use the turret to spray the top of the gunner's head to kill him, or just wait till it stops and kill it with the panzershreck panzerschreck. Or both. Rinse and repeat.
If at any time during this section you keep getting killed from behind (or from anywhere really) you can just throw smoke grenades at your feet to keep everyone off of you.
The good part about this section is you get a LOT of checkpoints.
Destroying the Tigers (with infantry!) (Suggested: Thompson and BAR, or any other weapon really.)
As soon as the artillery barrage starts again, head to the NORTH bunker (the little dirt on with a roof near the side). The tanks will later come over the wall about 30 feet to the west of this bunker, and you will 'spawn camp' them as they come across. Just stay in your bunker, jumping out when those perfectly aimed grenades come in. You don’t really need to worry about guys too much; you will only have 2 or 3 trying to kill you from the west side (where the tank comes from).
When you hear/see the tank coming up the hill, go out towards where it will come over the edge, plant a charge as it rolls past. BOOM. Easy. Return to your bunker, dodge a few more grenades, shoot a few more people, and do it again to the next tank... (You’d think they would learn...).
Congratz! You have finished a level everyone swears is the hardest, but I found rather easy...
Good Luck, and feel free to comment, message, question, denounce, debunk, and bash my (attempt at a helpful) guide.