On the level Tsavo Highway, once you get past the part of the map after you are forced to leave your vehicles, you will go through a tunnel out onto an open "freeway".
Immediately coming out of the tunnel you will see a huge pipe with two yellow ladders. Climb the second ladder. You should see a Regenerator power-up.
Proceed to your right onto a small platform ledge where the yellow chain link fence is located. Jump onto that ledge, crossing over the fence.
Directly behind you should be a large column that is holding up the giant pipe you were just on. Below that is a walkway.
Fall off the ledge onto the walkway. Walk to the end of that. You should see a hidden ledge with the "Tough Luck" skull on it. A few attempts may be requires to jump across.
Pick it up and there you go, the Tough Luck skull.
Enjoy!
