This prevents you from going back and farming the area on a high level character, but you must instead do it on a fresh character. If you really want to farm it, the you should create 4 different level 1 characters. Log into the first one and rush to Hillock. Kill him and hope a Unique drops. When it does not, log out and try again on the next character.
It tends to take a long time to get it to drop, and most people have 4 different characters that reach level 4 by repeatedly killing Hillock. Estimates to get the achievement is between 1 and 4 hours of intense farming, but you might get lucky and get it a lot quicker.
Getting Beginner's Luck during modified racesDuring certain races, such as Mayhem, you can get it a whole lot easier. The Twilight Strand zone will not spawn the most difficult modifiers, but it can spawn ghosts. What you should do it to get Hillock to low health and then kite him to get haunted by five different ghosts. This will make him incredibly difficult, but you should be able to get him down with a few deaths.
Farming Hillock with 5 ghosts will make him drop a Unique item a whole lot faster, and most people will get a unique item within 2-5 kills! This makes the achievement unlockable in under 30 minutes, compared to the regular hours of farming.