Amelia Black can nicely construct 2 Trading Posts in the meantime while the other units take care of the middle Trading Post. The reason I'm directing all those units a bit strangely in the beginning is because if you send them immediately to the Trading Post spot, they make a bit of a detour around a big rock. So this way they walk more straight towards it. Pretty strange in this level, if you completely exterminate the enemy, you still won't complete the level even though there's no competition anymore :)