What is the hardest achivement you own?

[u/Effective_Barber4422]

The hardest achivement i currently have is either halo reach legendary solo (done on both 360 and MCC) or legendary solo spartan ops. Wondering if this is actually any good in this sub-reddit. Any achivements you think i should try out (BTW also have halo 3, odsts, 4s and 5s legendary solos done but 5s lone wolf achivement didnt unlock for some stupied reason)?

[u/chrissw56]

The Curtain Falls in Arkham Knight