A Tribute to Skill (10 player)

If you've done the instance, or at least read up on Wowhead or even on Wowwiki, you would know the Faction Champions are only the third boss encounter in the raid. Even this achievement - Call of the Grand Crusade - lists all the boss encounters in the correct order too (right to left, top to bottom).

Unless you are considering when the patch was released with each boss on a lockdown period? During that time, each boss available was the only bosses you can do. After the fifth week, only then everyone was able to technically clear the instance cuz it was the week of the unveiling of Anub'arak, the Final Boss. With that, people were able to, not only clear normal mode 10-man and 25-man, but now have the privilege to run heroic mode 10-man and 25-man.

You are incorrect. Here is your correction:

So as far as i understand, you will need to down Faction Champions Anub'arak (The Final Boss) within 25 wipes/attempts.

The boss kill order goes the following:

1. Beasts of Northrend
2. 
   
3. Lord Jaraxxus
4. 
   
5. Faction Champions
6. 
   
7. Twin Val'kyr: Fjola and Eydis
8. 
   
9. and lastly, Anub'arak, the Final Boss
10. 
    

So, you need to clear the instances with at the max 25 wipes/attempts in order to get credit for this particular achievement.

To obtain this achievement, you must complete the 10-player version of Trial of the Grand Crusader (the new raid dungeon in patch 3.2, Heroic mode) while wiping fewer than twenty-five times.

This will reward 2 Trophy of the Crusade in addition to a weapon. Got it last night. You are required to full clear heroic 10-man TotC with 25 attempts remaining.