Freelance should be able to get these achievements completed to get the reward Eye of Decay. It gives a type 3 aug that improves damage for all abilities by 1%. With further achievements in other raids it can reach up to 10%. Seems to scale from 1% to 3%, 5%, 7%, and then 10%.
Based on the wins in the DKP site and Brenlaven's achievements it looks like Gifts Unopened was first achieved on 30 Jan 2016 (first win). Et tu, Kebaber was won on 11 Mar 2016 (if not earlier?). The only one left it Seven at One Blow at least for Brenlaven.
While this event is still a challenge, Freelance has proven to be able to get it down on a consistent basis. With the expansion beaten everyone should be eager to focus on getting the events down efficiently and trying some achievements when Furro calls for them.
Wins are most important but when Furro calls for going for achievements we all should work as best as we can to get them down for the benefits everyone gets from them.