Expert difficulty can be played at any time, but DO NOT do it as your first playthrough. You'll need some upgraded weapons (Specifically the Pistol, M1 Garand, STG-44, or anything you prefer) to do it.
And the difficulties DONT STACK. That means three playthroughs, so I recommend you do Expert LAST (for the reason above).
For most of the missions, it's fairly straightforward:
- Enemies DONT keep respawning until you move, like they do in COD.
- Stay crouched (Click LS) during firefights
- Take your time. If you sprint ahead, you'll just get shot.
- You can regenerate, but only for the health bar that has a fraction still left. Some enemies drop health packs, but dont sprint for them.
Also, for me anyway, the difficulty shifted from Expert to Normal for some reason. Don't "Continue," just select "Select Mission" and pick the next one. Even if you have NO STARS for expert (I had a few missions with 0 and still got the achievement), try selecting the mission after it on expert. If you cannot select it, you didnt beat the prior mission on expert.
Most of the game is easy, until you get to the bridge at the end of Operation Market Garden, then the difficulty skyrockets.
I'll list some tips for these harder sections:
The bridge is actually pretty easy, once you get a checkpoint before you go on the bridge (you'll see a pillbox and a turret gunner). Kill the turret gunner and run to the left side of the pillbox. Kill any nearby enemies, and make a MAD SPRINT to the objective at the end of the bridge.
Trust me, the bazooka troopers will aim DIRECTLY AT YOU, not near you. You'll probably die but you'll likely hit a checkpoint before you do. When you respawn it'll be just you and the tank you have destroy. Nice and simple.
Then there's Operation Varsity. Not a hard mission but a pain if you ignore the snipers. They can take off 1 1/2 of your health blocks with one shot! I cant remember for sure, but I think there are about 6 snipers in this level. You can see them via the flashes of light coming from their scopes.
The end of the mission, where you have to destroy the train, is also easy. You'll fight at least one Nazi Super Trooper (my term) with an MG42. The best way to kill him is work his face with any weapon. About 10 STG44 or 5 M1 Garand shots will take him down. Once he's down, thin out the remaining Nazi's and toss Gammon Grenades (the black round ones) into the holes on the train. Mission acomplished.
Then comes the Flak Tower. Like Market Garden, this one isnt that tough, but you need to do it in chunks. Take out as many AA guns or artillery before dying, then respawn and get the rest. You can also use Gammon Grenades on these instead of running up and planting charges on them.
Once you get the objective to "Get to the third floor" or something similar, the shit's about to hit the fan. This is (IMO) the hardest part in the game, phase one. If you don't have the M1 Garand at this point, pick up one. You'll be fighting at least 10 nazi's and 3 super soldiers. Move SLOWLY and stay behind the crates when you're outside the designated area.
Once you get inside, BEFORE you climb the ladder (there's a few Nazi's and a super soldier up there), look for health packs if you need them. The super soldier is at the back of the room up top, waste him with the Garand.
Then you take the elevator, and this is phase two. You'll see a super soldier and two nazis at the bottom. Ignore them, they won't fight you. Head left and into a lower red area, watch for the nazi up to the left.
Go down a little more and get the Nazi uptop. Here's where it gets HARD. You have to fight 5 or 6 super soldiers. The MG42 guys. Go back to the slant where you came in this area from and hide behind a wall on the left. Select the M1 Garand and push RS to equip the grenade launcher. BLOW EM TO HELL! Be careful not to pop out of cover too long.
Once there all dead, move up. If there's a guy you missed, get em with the STG-44. Looking ahead, there's an opening in the wall to your right. Make it through here and you're home free. But to the left, there's one more super soldier. Ignore him and make a sprint for the opening. He'll probably ignore you.
Climb the ladder, go through the sewer, and emerge at the top, outside the tower. Pick up the Detonator and blow the tower to hell. Mission Complete, and you're 75G richer.
Hope that helps with the harder parts of the game.