For both the French and British Campaigns Level 1 your unit must be the first one to open fire. Both of those Level 1 start of the same way where the player unit can get 2 shots off on the first enemy unit.
The second enemy unit will be close and approaching you on Turn 3 at this point you must retreat. See the attached images to see where you must retreat to and end turn. An example below illustrates the French Campaign Level 1.
At this point you should be able to kill the first enemy unit while the second enemy unit will waste two moves to get to you. Once again when it is your turn open fire on the enemy unit and repeat until you win