To get Idol of the elves you need to give Idril Bow of Eternity.
Bow of Eternity is located in the middle of Foothills map behind impassable trees.
To get access through the trees, you have to investigate Idril's sister house on the east side of map first.


You will get revenge quest by Idril to kill all thug rats groups around house.
Once you are finished with rats Idril askes you to get Bow for her, she grants you guidiance through the trees.
You can easilly go there, kill everything moving and take the bow.

After Idril gets her bow, there will be 2 new quests:
Staff for Guvi in the south edge of the map
and Potion from Lepricon for Idril in the north end of map.

It seems that for Eternal Love achievement you need to finish BOTH!
After you get that staff and kill lepricon, Cedric will tell you something like "do not play with girls, bla bla bla ..."
When you enter Dragon Mountain map, Cedric says that you can't be so cruel to three of you.
He informss you that Ant behind the bridge hasss ssssome precioussssss posssessssion that you can give to your beloved one (the girl you love more).

Once you kill Ant and get the ring, girls will get their "exclamation marks".

You can profess love to any of them, but for god sake do not try any other dialogue option! (game is buggy)
Boom. Achievements should be yours!