Hello,
while the Achievement Destruction Derby is bugged for alliance players, you can get the achievement with a little exploit.
You have to destroy the Siegeengine within a duel with your friend, so you and all players nearby get the achievement.
1. get Rank 2
2. build a Siegeweapon
3. exit the Siegeweapon
4. duel a friend
5. get into the Siegeweapon
6. let your friend destroy the Siegeweapon
even the player whos sitting in the Siegeweapon will get the Achievement. So have fun. ;-)