Tough Luck is a gold skull. It is the first skull on the third level, Tsavo Highway. As a gold skull, it offers an achievement and 150% bonus on campaign scoring. In return, enemies will have much better luck dodging your grenades, needles, vehicles and plasma bolts.
After emerging from the end of the first shielded tunnel and onto the Tsavo Highway, walk along the edge of the gorge to your left. Jumping carefully down onto the bridge support, walk around the edge of the supports to your left until you reach a stone ledge with this skull.
Enemies' improved ability to dodge seems to work best for them against long range fire from slow projectiles, like those from a needler. If you're used to delivering stickydooms at very close range, you shouldn't see an impact here. However, Brutes especially will dive quite a distance out of the way from needler rounds, making it difficult to get kills with the weapon unless you're up close and personal.
This is another skull that it seems you can use almost for free on Flood levels, as dodging doesn't seem to be very important to them, although assimilated human and elite combat forms will do it.