Don't mix elves with the archers...
There are some buildings at some stages which are damaged and need to be re-build by paying 100...re-build them...and now you can have 4 elves by paying 100 each...and when they die you can pay and re-have them...
while trying this achievement tactics are important...don't build strong archers or bombers or sth else around elves...because you have to make damage with "elves" only...
there are 2 stages which i used for this achievement
Silveroak Forest (there is only 1 building...so 4 elves)
Fungal Forest (there are 3 buildings...so 12 elves)
of course you can think Fungal Forest is better but you can die more at that stage...
put your hero between you and the enemies...also make soldiers instead of archers or the others...and make the soldiers only 2-3 up...if you make soldiers too strong then it may be hard for elves to kill before the soldiers...
good luck ;)