Here is the full list of guardians that you need to kill, there's no in game list to check which guardian you've damaged.
The achievement pops right away during the game. If you've attacked someone, but they disconnected before the end of the game, it still counts. However, if you've disconnected, nobody you've hit counts.
Gandolf
Nori
HalDir
Hildifons
Arathorn
Eowyn
Galadriel
Beregond
Ori
Thrain
Legolas Greenleaf
Gothmog
Runsig
Felgrom
Lugbol
Agandaur
Gollum
Wulfrun
Witch-King
Sauron
Ugluk
Mozgog
I've never seen the AI play certain guardians, these are Nori, Ori, Felgrom, Arathon and Sauron. The AI is usually playing Gandalf, gollum, Eowyn, Beregond, Thrain, hildifons and Wulfrun.