Preheat the grill to medium heat or about 425° F. - 450° F.
Lay out 8 squares of heavy-duty foil on the counter. Each packet will use 2 squares of foil
In a large bowl, combine the shrimp, crawfish tails, sausage, potatoes, and corn.
Drizzle with 2 tbsp olive oil, then season with salt, pepper, and garlic powder.
Toss all of the ingredients by hand making sure everything is thoroughly coated with oil and seasoning.
Divide the ingredients equally among 4 sheets of foil, making sure to place the food in the middle of each sheet. Also, be sure to place the shrimp on top of the other ingredients.
Sprinkle each with Old Bay and 1 1/2 tsp parsley. Then top each one with 2 tablespoons of butter and 3-4 lemon slices.
Create the foil packets by placing a second square of foil over the top of each foil packet, then roll the four sides in towards the middle, sealing the packets. Make sure each side is nice and tight.
Place packets in the middle of the preheated grill (sealed side up), and cook for 10-15 minutes until shrimp are cooked through and potatoes are tender.
Or you can cook these in the oven by placing the foil packets on a cookie sheet and cooking for 10 - 15 minutes at 425°.
Once finished, let the packets rest for 2 minutes, then drain any unwanted, excess juice from each packet.
Serve and enjoy!