Specialist is an achievement/trophy in the Metro: Last LightDLC, Developer Pack, awarded for completing the Shooting Range and Arena. Simply complete all 10 challenges in both sections (so 20 in all) and this should unlock. Both must be completed in one playthrough of the level, though you can save your progress using the switches nearby each section and come back to it later.
The shooting range is simple enough - shoot enough targets within the time limit to win the challenge. They get progressively harder, with more armoured targets that take more ammunition and time to destroy. The arena challenges are similar to the Tower Pack level in that you must defend a set area and defeat a certain amount of enemies without dying to complete the challenge. The enemies get progressively stronger, more numerous and with better weapons the higher you go, as well as having allied Mutants aiding them.