I don't know what I am doing wrong

[Petwin]

The rating tells you that their stacks are (much) larger than your stacks.

You're playing as a Mage, the hero type with the lowest leadership rating, so you will always be short on manpower. But a Mage is compensated with stronger spellcasting abilities.

It's for you to figure out how to use those abilities to kill of all those invincible enemies. Search the forum for tips on army setup and how to use the spells.

Or you can exploit the strenghts of certain units: (beware! this can be a game spoiler): go to the dwarf near the entrance of the cave on Bolo and buy Guard Droids and Repair Droids. Split the Repair Droids into 2 stacks. With this setup you can kill the strongest enemies without even losing units of your own.