New achievements unlock as you complete the old ones. We do have plans to add more achievements, but we've been waiting until we had time to revamp the achievement tab.
Looking at your gear, it appears that Natalie is a good choice for your DPS. You can always try out other Crusaders by getting them to the same price point and see what damage they do.
 "Crusaders of the Lost Idols"){kind=icon}
 "Idle Mine"){kind=icon}
 "The Several Journeys of Reemus: Chapter 2"){kind=icon}
New achievements unlock as you complete the old ones. We do have plans to add more achievements, but we've been waiting until we had time to revamp the achievement tab.