Enevhar Aldarion
Oh Wise One
I used 5 mortars for many maps like this and won every one of them at no loss with the exception of encounters that the enemy had priests, mist walkers (on the open side i.e positions 4 and 5) or mortar/frog. But even then, I replaced one mortar with a priest or Xbowman and won with only 1% losses to my troops
However, it would be much uglier outcome if my mortars were below 3* or I had to fight without fire phoenix and the sanctuary!
Mortars can still lose fights they should win because they have the largest swing from minimum damage to maximum damage of all 15 troop types. If your troops all get low damage output, you can get crushed in a favorable fight and if you get all high damage output, you can win fights you should lose. If their minimum to maximum damage range was in line with all the other troops, they would be a lot more useful.