You don't need a marinade, but I almost always marinate it with a mixture of half soy sauce and half orange juice.
When you say "chunk," do you mean more like a flat strip? When I buy it, it's usually cut in strips about an inch or two wide and 6 or 8 inches long.
Get the grill as hot as you can and get a good sear on the outside, and the inside will still be medium rare and amazingly juicy. I'm getting hungry...
(Note: when I cook it, it's not wagyu. I don't know how much of a difference the extra marbling would make)