No, no troops die on defense in PvP. They can die if they are donated troops in GvG after an entire army is killed 10 times, they are removed then.
Don't waste troops revenge attacking unless you can carry it through to the plunder stage and can take more than you lose or to make a point. Losing an army to kill one or a few unit that will pop back instantly , is nothing more than entertainment for the defender to watch, and maybe you to learn something from.
Retreat from battles the second you see you cannot win, unless you are trying to learn how the AI works in such situations, and quit as soon as you want. No need to lose units for nothing, if you know you are going to lose. Which should be apparent by the end of the first turn of all unit movement, If not before, on map set-up.