If you try to do a full melee or a forward thrust you will always die. Instead, try only moving the Wiimote as if holding an actual rifle, repeatedly making the motion of hitting the Nazi soldier with the butt of your gun.
During 'The Mace' mission you'll come to a part where you must keep the Nazi from reaching the trenches. Ignore the men and go to the right. Follow the path until you find a scoped FG42 leaning on a crate.
As a sniper always try to get into nooks, crannies, or other small places where you can crouch or go prone. Try to choose areas where you have a good view of your targets, such as tall grass or places where people won't suspect you're hiding (especially if you have a view of the enemy's spawn point). Also be sure to use your binoculars when you've think you've been spotted, that way you're less obvious to your target. Be sure to only shoot to kill. In other words, only take necessary shots so your target or their friends don't detect you.
During the part when you meet the French rebels you are asked to either go left toward the Flak 88s or through the house on the right. Choose left and kill all the Nazi surrounding the Flak 88 as well as the Nazi in the ditch leading to the next gun. Now blow up the Flak 88 along with the rest of the guns. Once you destroy all the guns you will find the house you would have gone though had you chosen right. Go inside and kill the Nazi waiting for you along with the rest of the enemies in the house. You'll then fall through the burning floor and meet up with your allies.
During the 'Fuel Tank' mission don't hold the controllers as indicated during the part where you wrestle the Nazi's rifle from him. Instead point the top end of both the Wiimote and Nunchuk toward the Wii's sensor bar.
While playing as the British during the mission Falaise Road there's a part where you have to clear out a building. Once cleared, head up to the highest floor and move to your right until you see a little room. Move to the left as soon as you get to the entrance to find three KAR98Ks and three MP40s. They respawn when picked up so choose whichever one you want and be sure to return when you need to restock on ammunition.
On the mission 'The Island' (second mission) there's a man under the second bridge you find with a scoped FG42. This handy gun starts with only 96 bullets but you get more ammo with each enemy you kill. After completing this mission you'll start the third mission with the FG42. Be sure to hold on to it if you want to keep it for all the following missions.
During the 'Saint Lo' mission you must first kill all the Nazi soldiers in order to capture the church. The best weapon for this job is the Thompson gun. Once all the Nazi in sight are dead, run up to the church and throw a few smoke bombs inside and immediately follow them up with a few grenades. This will allow you to go to your next destination, capture the church, and proceed to the next objective.