mocoso = n. brat

apestoso = adv. stinkin’

pendejo = n. single folicle of pubic hair. In spanish vituperism, it alludes to low intellectual capacities, i.e. idiot, dumb, stupid, moron. Under such context it becomes an adj.

de mierda =  of shit

As such, the correct arrangement would be:
“You stupid stinking piece of shit brat”