UPDATE (February 26, 2018): Guild Wars is now one of three alternating events (Guild Wars, Raid Boss, Invasion). This makes this achievement only obtainable during one of the three week period.
(For this achievement your guild needs to be registered for the particular week's Guild War.)
Each day, you have five battles against a guild. (Find them under the
Attack tab in the Guild Wars section in Guild)
The first battle is against a random "
soldier"
If you win, the second battle is against a "
vanguard"
if you win, the third battle is against a "
herald"
If you win, the fourth battle is against a "
champion"
If you win, the final battle is against a "
paragon"
The achievement will unlock upon
winning the fifth battle.
(Originally there was a bug that gave you the achievement even if you lost to the paragon. This has now been fixed, it seems.)If you are fighting a small guild, you might fight the same opponent multiple times.
If you lose a battle, you can't win the achievement that day, but you can try again the next day.
Just play with your favorite team and you'll get lucky, sooner or later.
If you're really struggling (which might happen if you are in a guild with a lot of advanced players - you will be teamed up with more advanced guilds), you can try leaving your guild and starting a new one. The first week will probably be the easiest. There will be a lot of newbie guilds to fight.
If you're looking for tips on best teams, check discussion on the Internet, e.g.
https://steamcommunity.com/app/329110/discussions/0/62071236...You can slightly improve your odds by purchasing bonuses for each week's war in the Sentinels tab. It is costly and only valid for a week. It might be a better idea to invest in your troops, leveling up kingdoms etc.