I will describe to the best of my ability of how to get those pesky Soul Orbs for the Death God. Please note, there will be slight spoilers so be weary.
Guide to Farm Soul Orbs
All credit goes to ZylpheRenuis!
Gorgon Statues
There are three breakable Gorgon Statues (Not bosses) that damage your character each time you attack it. With every Gorgon Statue damaged and broken. A Multiplyer is applied for Soul Orbs at the cost of lower health pool. Reminder, after every death these statues respawn so you have to destroy them again in order for the maximum output of Soul Orbs to obtain.
The First one is easy to find As soon as you activate the *Lift* having it coming down. You can go right into the next Screen and destroy a blue Beetle. At the end there is a stepping block. To the left of that block you can jump on instead of jumping on it. Press down in the corner to decend into a hidden area with an inactive teleporter. The next room to the right is the first Gorgon Statue. At this point you can only grind for a bit of Soul Orbs so go get that 2x gain. Be careful of proceeding through the game. The lower health pool from destroying the Statue will be troublesome.
The Second one is in the Mechanism Ruins. Refer to the map pictured as I painted the exact spot green. Beneath the area is a ladder that is in the upper left of the area beside a Skeleton, a Bug and a scouting turret on the right wall. At this point you have gotten a few deaths in and upgraded your health pool. You should be able to make it x4 now if you destroyed the first one.
The third is in Subterranea 1 as soon as you find the lift in the area below Gorgon Tomb. Go left of the lift and the last one is right there. Destroy that with others to get the max output of 8x Orb obtention.
Extra: Hidden in the Gorgon Tomb via accessway back from Mechanism you have to use a teleporter to reach into a area in the middle of the map. This is where you get Amulet of Sol. (You might need to have some Red Keys, Blue Keys or White Keys to access it. I don’t remember.
The location of it is marked in Green.
Golden Statue of the Bald Headed Unfriendly and the reoccurring Gold Knight mook that gives 7500 max Soul Orbs in Tower Roots located at the far east of the area. In order to access it you need to have Bram and Zeek have their extra skills located in Cathedral and Serpent Path. The Block Ring and Morning Star. Then have your skilled wall climber jump the cliff into the golden room. Once there stand still in the middle and the room will shine and sparkle. The Bald Headed statue will be unfriendly and split apart spawning a extremely tough Enemy Gold Knight that when unprepared can easily kill your characters dead.
Use Bram, Algus, or Arias to kill him easily and earn your 7500 Soul Orbs upon vanquishing. Upon returning back into the room that statue will still be unfriendly and spawn more of the mook so the process is repeatable.
Just make sure you destroyed the 3 Gorgon Statues else you won’t get the full amount of 7500. This is BY FAR the fastest way to grind out the rest of your characters in game.
The room you marked as green for the Amulet of Sol section is incorrect. You marked the first of three locations where you can find and speak to the Old Man that you can only access with Algus (the wizard).
The actual location of the Amulet of Sol is 4 rooms left and 5 rooms up from the room you marked.
To get there, you’re gonna need to have:
Red Gorgon Eye – Beat the Gorgon Tomb’s Boss – Gorgon: Tauros (first boss), Opens Red Gorgon Eye Doors
Blue Gorgon Eye – Beat the Mechanism’s Boss – Gorgon: Volantis (second boss), Opens Blue Gorgon Eye Doors
Griffon Claw – Found in Hall of Phantoms, for Kyuli to make consecutive wall jumps
Banish Spell – Found in Gorgon Tomb, for Algus to kill the ghost enemies
One Blue Key
Starting from the Gorgon Tomb 2 Elevator, head right two rooms (the Tauros Boss Room, and the entrance of Mechanism), then climb up to the moving platform and jump to the left ladder and head up.
Continue climbing up the ladders until the top of the room, where you can go right and unlock a white door. Go left instead, the wall is fake.
Make sure you’re Kyuli for this part because you need to wall kick to reach the ladder from the ledge below it. But first, grab that Blue Key at the bottom of the room. Climb up to the next room.
In this next room, head left (if you go right, the wall is fake again and you will find a room leading to some orb cubes, two Pils (pink floating enemy), a Rotton Skeleton, and a blue key door leading to the save/bonfire room two rooms north of the first save room you come across in Mechanism), kill the skeleton and use that Blue Key you just grabbed to open the Blue Key Door to continue left into the next room, which is a warp room, dont use it for now, just ignore it and continue left into the next room.
This room will have (probably your first) Red Key on a pedestal. Grab it and continue going left through Blue Gorgon Eye Door into the next room.
From here, you can head up and open a shortcut to the rest of the Hall of Phantoms, or continue down to the next room where theres a button switch on the left side that loosens and drops the huge rock at the top of the shaft your next to. Step on it and drop the rock to open the way up.
Now use Kyuli to wall jump with the Griffon Claw all the way up the left side of the shaft until you reach the top of the next room where there is a Red Gorgon Eye Door and a Blue Gorgon Eye Door blocking the way left. Kill those fairies while the doors open for you and then continue left.
Depending on your health, either make a mad dash for the Amulet of Sol or use Algus to kill the three Wights that float around the room.
Bam. You now have the Amulet of Sol and now the number of orbs you collect has permanently increased. Now just backtrack to the shortcut you did or didn’t open, or the Warp Room or wherever else.