Increased loss of soldiers while attacking the level 12 tiles in RoC, how to improve that loss?

The losses you will take will depend initially on having enough 'Loyalty' from the Coalition base camp buildings. Once this is above the threshold for the tile level then you will not take extra damage. After that it will be down to the strength of your legion (number and level of troops and your heroes skills) and aided by the levels of your Assault and guardian fortresses to give you extra buffs.