Toy Soldiers

Not as hard as the achievement makes it sound, the only major difference with the "Elite mode" is that units do not automatically fire, therefore you have to kill every soldier manually. Simply choose the first level and set up some basic defences, upgrade them as and when and just keep switching between unit to kill oncoming soldiers.

You can do this pretty much on the first level. Elite mode and upgrade your Machine gun to level 3. That can easily take care of anything the computer throws at you on this level.

If any DO get past. Switch to a gas thrower at the bridge to your toybox to finish them off.

With respect to everyone who posted a solution before this (esp FearlessDonut, since that sounds difficult in and of itself!), can actually get this one really easily on the first level, building only two defenses.

I did it with one gas thrower at the opening of the base, which takes care of all of the waves save the last two. Placing it here, I got every infantry troop before they made it to the wall into the base, and even if any do manage to get over the wall, easy enough to turn and take them out still in the first causeway. In the large emplacement area right behind it, place a machine gun and upgrade it to level 2. The machine gun almost too easily takes care of the cavalry that compose the next to last wave, and the lone tank that shows up as the final wave.

While getting this achievement you may as well get the Fort Vaux ration ticket too. You need to complete the mission using only the biplane/triplane. By choosing elite you get 40 points on your toy box instead of 30. Also you can only use one unit so you dont loose anything there. Just complete the level and wha-la success and happiness!