Here is the full list of all 29 Guardians of Middle-earth achievements worth 400 gamerscore.
Win with a Warrior-Class Guardian in any Battlegrounds Match
Upgrade 3 Towers in any single Battlegrounds match
Win with a Tactician-Class Guardian in any Battlegrounds Match
Earn 5 Accolades from any single Battlegrounds Match
Win with an Enchanter-Class Guardian in any Battlegrounds match
Achieve a Kill Streak of 3 in any Battlegrounds match
Kill or Assist a Guardian from each Class once in any Battlegrounds Match
Build 6 Soldier Upgrades total in any Battlegrounds matches
Use an Ultimate Ability to defeat an Enemy Guardian in any Battlegrounds Match
Reach Level 14 in any Battlegrounds Match
Fall on the Battleground
Achieve an Assist Streak of 5 in any Battlegrounds match
Achieve a Multi-Kill of 2 in any Battlegrounds match
Destroy 5 Soldier Upgrades total in Battlegrounds matches
Win a Custom Match
Win any 3-Lane Battlegrounds or Skirmish match
Win with a Striker-Class Guardian in any Battlegrounds match
Win with a Defender-Class Guardian in any Battlegrounds match
Use a Tier-4 Command in any Battlegrounds match
Use 4 Potions in any single Battlegrounds Match
Earn 20 Guardian Kills in any single Battlegrounds Match
Earn a combined Guardian Kill/Assist Streak of 8 in any Battlegrounds Match
Capture 15 Shrines total in any Battlegrounds matches
Earn 50 Accolades total in any Battlegrounds matches
Play as every Good Guardian once in Battlegrounds matches
Play as every Evil Guardian once in Battlegrounds matches
Damage every Guardian once in Battlegrounds matches
Win 25 total Battlegrounds Matches
Win 100 Battlegrounds or Skirmish Matches