Human Effigies can be found from Starting Gift, Static Locations (corpses), Merchants (All merchants have limited supply), and NPC's (both given and dropped when killed).
The term 'farm' usually implies an unlimited resource, as you pointed out. As of the latest patch, the Company of Champions covenant allows infinite respawn.
These are enemies that drop Effigies:
- Attack Dogs.
- Skeletons [not in Undead Purgatory]
- Turtle Knights.
- Sentinel (Sword/Shield) and Sentinel (Great Sword)
Always remember you can farm 6 effigies off of the Old Firekeepers and 3 alone off Housekeeper Milibeth in Things Betwixt.
After unlocking the Shrine of Winter, it is possible to farm Smooth and Silky Stones. While not directly farming Effigies, these have a high conversion rate to Effigies at Dyna & Tillo (Drop trade crow), although this is random.
There is however a way to restore humanity consistently.
In the Shrine of Amana, above the last bonfire, there is a Shrine with a praying Milfinito. After you have 'rescued' the two stray maidens (one outside the Demon of Song room, another behind the Embedded Door in Drangleic Castle) you can pray at this Shrine at any time to regain humanity. This is free of cost and can be done a limitless amount of times.
There is an easier way, although it seems so easy that it will be patched.
This requires the Ring of Life Protection, which can be found at the No Man's Wharf. This ring prevents loss of your Humanity and souls on death, but breaks afterwards. This ring can be repaired for 3000 at the blacksmith giving a constant 3000 Souls per Human transfer rate. A +1 Ring of Life Protection (Also prevents petrification) can be purchased from Vengarl in the Shaded Woods.
I'm a blue knight and yes I have to wear my ring for you to summon me and you must be in human form with your ring on to summon me. I've been summon from random places all across the map, even to areas I hadn't yet been too. The only place I wasn't summon to was the bell tower as that was a pvp area for the protectors of the bell covenant. If you were in that area, if you weren't human, if you weren't wearing the ring and one of us wasn't wearing our ring, that is why you couldn't summon one of us.
As for summoning and invasion
The way it worked in dark souls 1 and dark souls 2 are different to make it more fair. It depends both on soul level and soul memory. Your soul level is what level you are while your soul memory is based off of how souls you spent on your armor weapon and levels. You can find it in your player status menu. It's to help fight against over powered people fighting under powered people.
Hope this helps and praise the sun.
Best Answer
Unfortunately for you, It can't be done in that order. For it to work properly you have to
This is fixable with a bonfire ascetic. Using an Ascetic on the "King's Gate" bonfire respawns Nashandra and the Throne Watchers. This unfortunately also puts the fight (including the one with Aldia) into NG+.