Since Ulduar and ICC achievement mounts started in Wrath of the Lich King, all raid patches added one Achievement: Glory of the Raider, which can gain some achievements like Glory of the xx raider when completing all boss achievements.

The Glory of the Draenor Raider will get a reward: Gorestrider Gronnling after complete all boss achievements in 2 raids: Blackrock Foundry and Highmaul. It is hard to get this achievement without a powerful team. 

The hardest achievements:

1. Ashes, Ashes...:
Defeat Blackhand in Blackrock Foundry after catching 20 Falling Ashes on Normal difficulty or higher. All Ashes drop outside of platform，you must jump out of platform to reach ashes and return. It needs these classes who have teleport ability: Monk's Transcendence, Hunter's disengage, and Warlock's Demonic Circle, etc.
2. Stamp Stamp Revolution:
Defeat Hans'gar and Franzok in Blackrock Foundry without any raid member being crushed by a stamping press on Normal difficulty or higher. All players must highly concentrate in the game. As long as one player makes a mistake, the achievement will fail.
Another achievements are simple a lot, but also need some skills. However, it will be much easy for the professional team.