Dark Souls II SotFS [Benhart of Jugo Quest and achievement]

In order to obtain the achievement of Benhart of Jugo questline in Dark Souls 2 you must do the following:

You need to summon him at least for three boss fights and he must survive. You can summon him to assist you in the battles against Prowling Magus (Brightstone Cove Tseldora), Looking Glass Knight (Drangleic, King's Passage), Elana, Squalid Queen (Dragon's Rest, DLC Sunken City) and, finally against the Giant Lord (Memory of Jeigh). There is another spot where you can summon him (Throne of Want) but, this does not count to get his achievement.

It is also important that before going into the Memory of Jeigh you encounter him at the Memory of Orro. You need to complete his dialogue (I did not) and, then the summon sign will appear at the memory of jeigh.

You can also use a bonfire ascetic and battle the same boss with him three times.

After defeating three bosses with his assistance speak with him again at the memory of Orro and you will get the achievement and the following items:

Moonlight Greatsword achievement
Bluemoon Greatsword
Benhart's Parma
Benhart's Armor Set

Finally do not confuse (as Benhart did) the bluemoon greatsword with the moonlight greatsword. The Moonlight Greatsword is obtained by trading the Old Paledrake Soul (Duke's Dear Freja in NG+) with Weaponsmith Ornifex.