Here is the full list of all 15 Guardians of Middle-Earth trophies - there are 13 bronze, 1 silver and 1 gold.
Win 25 total Battlegrounds Matches
Win with a Striker-Class Guardian in any Battlegrounds match
Win with a Defender-Class Guardian in any Battlegrounds match
Win with an Enchanter-Class Guardian in any Battlegrounds match
Win with a Warrior-Class Guardian in any Battlegrounds Match
Earn 50 Accolades total in any Battlegrounds matches
Earn 20 Guardian Kills in any single Battlegrounds Match
Capture 15 Shrines total in any Battlegrounds matches
Earn a combined Guardian Kill/Assist Streak of 8 in any Battlegrounds Match
Earn 5 Accolades from any single Battlegrounds Match
Win with a Tactician-Class Guardian in any Battlegrounds Match
Kill or Assist a Guardian from each Class once in any Battlegrounds Match
Reach Level 14 in any Battlegrounds Match
Use an Ultimate Ability to defeat an Enemy Guardian in any Battlegrounds Match
Fall on the Battleground