Hunt: Showdown

This achievement is again pretty much self explaining. You'll have to reach tier 3 of the bloodline which is the equivalent to level progression.

Bloodline tier 1: rank 0-33
bloodline tier 2: rank 34-66
bloodline tier 3: rank 67-100

The is no easy way - the best method to get there fast is to kill Hunters, find clues and extract two bounties. As soon as you know there is no one else on the map (no other hunters remaining) you should try to kill as many monsters as you find, e.g. Grunts, Hellhounds, Hives, etc. - this will give you additional xp.
Keep in mind: only the xp counts towards this, your money / bounty is not important BUT you still need to play the objective to get as much xp as possible (e.g. banishing the boss, etc.)