For an opening level, Semper Fi has a few sections that can catch you out. After you escape and begin to work your way through the beach huts keep your eye out for a Marine going hand-to-hand with a firey Japanese soldier - save him and you unlock an achievement. The next tricky part is the ambush in the jungle, which you can beat by taking cover behind the burnt hut. Finally, the end requires you to plant explosives then escape on the boat. Don't mess around here trying to kill all the enemies - just sprint for the objective.
Second level in, and there's nothing too tricky here. It's easy to forget that you have unlimited rocket strikes, so if you're struggling to get past one of the machine gun nest sections, call one in and push forward immediately afterwards. There are a few places in the middle of the stage where you're ambushed by banzai chargers coming out of the grass, so be prepared to melée. Also, when you're picking your way through the tunnels watch out for half-dead enemies dropping grenades at your feet.
This level really starts to get going once you're out of the jungle section. Use smoke to progress through the Japanese barricades until you see your buddy drop his flamethrower. Once you've collected the weapon keep it! Burn everything that looks flammable: chances are you'll kill enemies hiding in grass and trees before they fire a shot (and you'll scoop an Achievement too). At the bombed-out air terminal, head to the left and push inside using the 'thrower. Head upstairs as soon as you can - you don't have to kill everyone here. Next is the tank charge. Stick to the friendly tank until it gets taken out then sprint towards the objective, where you'll find a Panzerschrek and plenty of ammo.
Finally, you're assaulting the AA guns. Again, use smoke to cover your approach and use the flamethrower liberally. Once you've take the first AA emplacement you can use it to take out the others without going near them. The level ends with a fixed machine gun battle. Mow down the troops until the tanks arrive. This is when you need to get on the nearest AA gun and take them out.
A good tip for the first part of this stage is to line up the paired guards to take them both out with a single bullet. No-scope the dog when he gets close and then gun down the final Nazi. The toughest part of the stage is the one-on-one sniper battle. Ideally, you'll take him out when Reznov draws his fire, but if you don't you need to listen out for where he is.
He'll try and fool you with a decoy (a helmet poking out from the bottom of the window) so ignore this and wait for him to appear. As a rule he usually fires every 8-10 seconds, so time your aim-steadying accordingly and you should nail him. The rest of the stage is reasonably easy. Keep low when you're crawling through the burning building, and don't miss the superior German sniper rifle before you climb onto the roof in the second half of the stage.
Their Land, Their Blood
This level is reasonably straight forward until the first tank-killing section. There's Panzerschrek ammo on the second floor of the house, which gives you enough to kill three of the four tanks. The final tank is in the barn at the top of the hill, and there's extra Panzer ammo in the nearby house for that. Once you've been taken across the map on the friendly tanks it gets tougher. Try and find a Paratrooper rifle (FG-42) - it has a good balance between power, accuracy and rate of fire - and a Gewehr-43. Now flank to the right side of the map and work your way forward. Pay attention to the snipers in the towers and try to take them out as you go. Keep pushing forward and your tanks will eventually arrive and successfully rout the Germans.