What should I do with the excess troops I train daily to complete the daily challenge?

As you upgrade buildings and hero skills you will be able to place a great deal of troops in your legions. You will need these for battling in the various events in the game. It is a good idea to have a lot of troops as they will be wounded or killed in battles.

If you do have troops you want to get rid of then the only way is to let them be killed or wounded in battles. If they are wounded and in the medical tents then you can dismiss them which removes the permanently.