from the experience i have i found out that some type of units are better than other to attack de troops on the map.
this is what i found so far:
to attack devil troops, beserker only lose at most 2 soldier and most of the time none.
i havent tryed against the knigth cause they are the best.
to attack harpy troops the mounted crossbow are very good the same result as the beserker for devils but just for airwalker and wingless.
Note: the attacks made with mounted crossbow were lvl 1 and 2. with better troops probably you can attack Skyclaw and Wind Huntress without losing 12 troops.
the attacks made with beserker were with lvl 2 and 5. on Arcane Channeller(the weaker), Slave, Spellsword,thralmaster(lvl5 beserker).
if any one can find the best troop to attack the merman post it here.