Here are some tips. If your pinned by a machine gun or two use a smoke grenade to find cover closer to it. When throwing a frag grenade conisider the timer at all times. Just because it disapeared after you threw it it doesn't mean it doesn't exist. It will still explode when the timer would've went up. Always snipe as many enemies as you can from a safe spot before trying to enter the area. You may find yourself pinned! Reload everytime your not fighting. Always have 1 shotgun/sniper rifle and 1 machine gun. The MP40 makes a good machine gun because ammo is easy to find. Try to get good at using German rifles because German ammo is common. I hope my tips benefit you because using these tips helped me beat the game on hard level!
If you're running low on M1 Grand, Thompson ammo etc your allies might have some, look for the ones whos names come up in orange or red - this means they don't have much health left so melee attack them with your gun and take the ammo, don't worry as more allies usually respawn at check points or regroups.