They have massively made this achievement easier!!
Now, all you have to do is get in a Stronghold Siege match--sort of. That is the hard part because either everyone playing Siege is already playing, or there aren't many playing.
Only try to get this on the weekends! There's so few people playing, that the weekend events are your best chance to find more players, unless there's a super event going on.
When you finally get into a match, you're golden! Except, it's GLITCHY, and may or may not unlock!!
All you have to do is finish the match, you don't have to score a certain number of points or anything, as the achievement description now reads:
"You fought for your guild in a Stronghold PvP match."
In my achievement winning match, I did pretty bad. With my Haste Cleric I got slaughtered by dedicated PvP'ers, escorted some horses, built some ballistas, and that was it.
Finished with 115 points, my team got decimated rather quickly, and the achievement popped at the end.