Note: The achievement automatically completes when you kill the last enemy. Watch the popup in the bottom right corner for the achievement, don't go through the second door near the end before you see the popup.
There are two Communist soldiers in the back of the tunnel the player starts in.
The Communist traitor must be executed, the player does not have to do it themselves however, if wait long enough he will be killed.
The injured Communist soldiers must be killed, be warned the often make noise when killed. The guard can be killed with a throwing knife, and then the injured soldiers won't alert anyone. If you kill them with grenade, no moral points will be lost.
Note that there are some Nazi soldiers under the main bridge (on their side) which may or may not move to the main bridge during the fighting. The player can access this sub-floor via a flight of stairs. These are the same ones that would be encountered if the player enters by the pipe entrance.
The Nazis on the rail car also need to be killed, a grenade or two will make this task easier.
The Communist prisoners must be killed, note that this will result in negative Moral Points, to avoid this, take cover behind them and the Nazi soldiers will eventually throw a grenade that will kill the prisoners without the loss of a Moral Point. [Confirmation request.]
Freeing the prisoners will make them invulnerable and thus you will lose the achievement.
In the traditional sense, a hedge-hopper is someone who flies an aeroplane very close to the ground, avoiding objects as they appear. The relationship between this achievement and actual hedgehopping in this sense is not very clear, though "hedgehopping" could also refer to the fact that the player is not siding with either side, Nazis or Communist, and thus hopping the proverbial "hedge".
This is best earned if the player is not seeking the enlightened ending, as killing everyone in the level will not garner positive moral points.
Despite the players will gain negative Moral Points doing this challenge, they can use this opportunity to complete other weapon-related achievements.