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