Merge & Blade

This achievement will stack with the one for merging eight units at the same time if you did not already have it. I like the following strategy as it is easy to do and does not rely on starting early in the campaign.

Done on the first stage of the challenge mode. Keep merging until you have eight Footmen close together and then purchase the first Academy upgrade. Place one more unit and the Footmen will merge. I also went with two upgrades for the footmen to keep them alive and swapped weak ones to the back of the formation to avoid losing them in battle.