My very first time I did the easy battle I lost at least half of all 8 sellected units ! The second time I only lost 1 or 2 light units and each next try I learned by "try & error" . Now I do not loose any unit on the easy one ! It's a matter of time to "learn" handle the units and learn their skill, weakness and strengths ! It makes a BIG difference when you attack an enemy unit with that kind f unit that has a "bonus" versus (vs) it ! Also try to make advantage of the battlefield's terrain /area (bushes,, forest, plain , ... )
Sometimes it's good to bring the light units first forward (together = swarm skill!) and form a wall, place the rangers behind that wall ...
Or try to block the best fields for your enemy, so they can't stay on it ! Example : an artillery unit has a big advantage staying in bushes (hides!!), so block those bushes if possible ! Heavies has a defense & attack bonus on plains ! Try to place your lights in such way that a heavy have to go on a hill or whatever to be aible to attack one of your units ...
Short :
Light units are good vs Heavy & Fast
Heavy units are good vs Fast & Ranged
Fast units are good vs Artillery & Ranged
Ranged units are good vs Artillery and Light
Artillery are good vs Light & Heavy
Example : Do NOT try to kill an Artillery unit with Light units, you gonna to make hug losses !!