The Friend of the Trees achievement reads "Defeat Maarselok on his perches without defeating any Azureblight Lurchers in Veteran Lair of Maarselok." Y
our group should carry on with the fight normally and have your tank take the Azureblight Lurchers that spawn (only two will spawn) and tank/kite them around until the end of the fight.
If one dies, you will need to reset the fight.