To make your army most effective against the enemy, first find out what type of unit is their
special unit. Your army should be comprised of many units that have strengths against that 
unit type (If their special unit is a mounted one, then your army should be filled the highest
level spearman you can create, etc.). In the same way, your army should be filled with units 
which have strengths against the other unit your enemy likes to attack you with most often. 
The rest of your army can be filled with whatever you like.