There are Bosses that may be encountered. Each of them must be defeated in the same playthrough.
Play Endless mode under the highest difficulty. Do not take any food at the start. Allow zombies to attack you whilst scavenging to reduce your health. When it is low, begin travelling again until your character dies. Start a new game in Endless mode and travel until you find your tombstone.
You will randomly find tombstones as you travel. Tombstones found across all your playthroughs will qualify for the total required for "Grave Robber". It is also possible to repeatedly find your own grave in Endless mode, as described in earning "Deja Vu".
This requires some luck, as the items will appear during scavenging are random. Scavenge at every opportunity and collect food items (shopping carts, tins of food, chocolate bars) first. Do not purchase the upgrade that increases the amount of food you can take from scavenges.
Purchase or find five upgrades for the car and have them installed at the same time. Safe locations occasionally will have an auto shop that sells upgrades. Check all of them during your trip and save $100 to $200 to purchase the upgrades. The available upgrades are random at each shop. Parts can also be obtained random during random encounters during your journey.