To get the redeemer you have to last one round ONLY with the retriever. U cant kill the zombies with anything else. As soon as the round ends wait till a new one starts and then leave the bridge. When u are on the island again u need to survive one round. When you've done that you can go down to where the retriever was, enter afterlife and take your redeemer. For the skulls they are a bit tricky sometimes. At the one in the docks on the lamp near jug, stand in the corner left of the window to the opposite side of the jug and look up. Then u want to jump and while in your jump throw the retriever. I dont know which skull has been troubling you but this has always been troubling me. Hope this helped :D