After reading through my previous topic, it seems that my game had glitched regarding the Lucatiel questline, she will disappear from Aldia's Keep if you exhaust her dialogue there. So in order to avoid this happening, be sure to talk to her there only after you have her survive 3 boss battles.
As for the rest of these, they can be all gotten during postgame, so they are not missable. You can summon an NPC as many times as you need, as long as he/she's alive when the boss is down, it will count. It doesn't have to be 3 unique boss fights, you can repeat a certain boss if you have an easier time dealing with that boss.
Moonlight Greatsword: Inherit Benhart of Jugo's equipment.
You need to summon Benhart for 3 bosses and have him survive all 3 fights. You first find Benhart waiting by the entrance to Shaded Woods.
After unpetrifying Rosabeth, talking to him will cause him to leave for the Bonfire after the dual Dragon Rider fight at Castle Drangleic.
After that he'll be at the spot right after the main fight with the Pursuer, near the crow's nest, afterwards he'll be inside the Memory of Orro, where he will stay until you're done with his questline.
Here are the places you can summon him.
-The camp bonfire at Brightstone Cove. (Before the Prowling Magus fight.) (You have to have talked to him after unpetrifying Rosabeth for him to be available here.)
-Right outside the Looking Glass Knight battle. (Have to talk to him at Drangleic for him to be available here.)
-Memory of Jeigh down the stairs from where you start. (Have to talk to him at Crow's nest for this one.)
-Right outside the Throne of Want for Throne Defender/Watcher/Final Boss. (I've heard the final boss doesn't count, but according to Nayiko, it does indeed count.)
After summoning him for three fights and having him survive, talking to him inside Memory of Orro will cause him to give you his equipment.
Holder of the Fort: Inherit Captain Drummond's equipment.
Captain Drummond can be found inside the Memory of Vammar. To cause him to give you his equipment, you need to defeat the Giant Lord inside the Memory of Jeigh.
You can defeat the Giant Lord before meeting Drummond, it will still count and he will give you his equipment upon exhausting his dialogue.
Lucatiel: Inherit Lucatiel of Mirrah's equipment.
Basically the same as Benhart, except you can talk to her in her 3 starting areas in any order. (Those being Lost Bastille, Earthen Keep and No Man's Wharf.)
Here are the areas you can find her in:
-Lost Bastille straight ahead from the Towers apart bonfire.
-No man's Wharf right by the bonfire.
-Earthen Keep at the bonfire right after the Covetous Demon.
-Black Gulch, near the first bonfire, if you light a torch and hug the edge, you should see a ledge, drop down there and she'll be waiting. (She will appear here after talking to her in the previous 3 areas.)
-Aldia's Keep, right by the 1st bonfire. (She will appear here after you talk to her in all other areas. Do not talk to her here unless you've already defeated 3 bosses with her. She will disappear and so far, no no has any idea where she travels to.
Here are the bosses you can summon her against:
-Versatile Sentry at No Man's Wharf. You'll find her summon sign at the switch that opens the shortcut through the 2nd level of the Wharf. (Have to talk to her at Wharf for it to appear)
-Lost Sinner at Sinner's Rise. You'll find her sign right at the elevator that drops you to the Sinner fight. (Have to talk to her in Bastille for it to appear)
-Smelter Demon at Iron Keep. Sign is right outside the boss fight. (Have to talk to her at Earthen Keep)
-The Rotten. Her sign is right next to the hidden bonfire at Black Gulch. (Have to talk to her in Black Gulch)
After winning 3 fights with her surviving (Not before.), talk to her in Aldia's Keep and she will give you her equipment.
Smith for Life: Inherit Steady Hand McDuff's equipment.
After giving McDuff the Dull Ember found in Iron Keep, light the torch in his room to have him move away from the chest he's sitting on, after that, spend a total of 50 thousand souls on his wares while also having 10 thousand of those souls go into weapon infusion.
Note: This trophy has had many conflicting reports. Some people reported getting it after simply spending 14 thousand souls at McDuff. In my experience, it took 40 thousand souls, and then an additional 10 thousand that went into infusion for him to give me his equipment. It could simply be bugged, but for the moment, that's what triggered my trophy.
Garroulous Miser: Inherit Laddersmith Gilligan's equipment.
After Gilligan moves to Majula, spend 12 thousand souls to build his biggest ladder and he'll reward you with his equipment.
Reflections on Disembodiment: Inherit Head of Vengarl's equipment.
You can find the Head of Vengarl in the Mist Forest part of Shaded Woods, simply exhaust his dialogue and he will reward you with his equipment.