this achievement is easier than the tutorial. Although I would still recommend doing the tutorial first to get the basics of the controls, it only takes about 5 min to complete if u put the settings right.
select skirmish
select sandbox
choose your team
change difficulty to easy with "x"
choose medium or small army size if u are uncomfortable with larger armies (the smaller the army the quicker)
push start to begin
select a skirmish battle from the map
win the skirmish (FLANK FLANK FLANK)
achievement unlocked!
if you need help then comment below or msg me on xbl or here, i have helped several other people work their way through this game. hope this helps!