Challenge Conqueror: Silver

As a player who has completed all the gold achievements, I would say this one is easy to get. You absolutely don't need to cut corners, pull recklessly or perfect your setup to beat the silver timers. Play carefully, use all the CC you need and take all the mana breaks you want, and you can still get this one with much error margin. Just don't wipe too much =)

Blizzard notification on the Phoenix mounts come WoD:

"The Pandaren Phoenix mounts awarded for a full set of Silver medals will become account-wide. If any character on an account has earned a full set of Silver medals, all four variants of the Pandaren Phoenix (Crimson, Ashen, Violet, and Emerald) will become available to all characters account-wide."

Official post can be found here.

When you complete the achievement, you will receive 1x [item=90045] in the mail. This can be redeemed for one of the following mounts:
[item=89154]
[item=90710]
[item=90711]
[item=90712]
It is speculated that you will only be allowed to choose ONE of the mounts, so pick your color carefully.

To redeem, visit NPC [npc=66973] located in the Vale of Eternal Blossoms, outside the Seat of Knowledge.

Also note: Completing the achievement Challenge Conqueror: Gold will also award this achievement if not already earned.