tested this out at deer camp this past week:
Sliced 6 mallard's (12 breast pieces) into small strips.
Made a mixture of 1 bottle of Soy Sauce, 3 tablespoons of minced garlic and some ground pepper.
I put the duck slices in the marinade for 28 hrs (due to time constraints--12-18 hours would probably suffice) then put some tin foil on the grill so they wouldn't stick and grilled about 5 minutes on each side to Medium.
Super tender with a hint of soy sauce and garlic. Boy were they ever good!
The boys at camp had never had duck before but they'll be back for more.