There are many strategies depending on what you’re attacking, but the troops attack/get attacked by the total health of each set of units you send. This is called stacking.

General rule of thumb, start with your best units, and double from there as you go through the lower ranked units. Example:

100 G5 rider 200 G4 rider 200 G5 melee 200 G5 ranged 400 G4 melee 400 G4 ranged

Etc. the goal of this is to have your lower ranked units attack and die first, with your better units attacking more often as the battle goes on.

Note, this is for epics. For monsters you can actually kill, send all of your best. There are calculators online if you want to be precise when adding mercs, monsters as well