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