1. Take the Old Hero Archstone from the Nexus.
2. Back track to the slug caves.
3. There are white lights that will explode when you are near them. A good shield will help you survive a blast from the front but if it explode behind you, you may get killed. With less than half your equip weight used, you can simply run past them and avoid getting hit.
4. Run through the first narrow path, be careful not fall into the slugs waiting below.
5. Once pass the narrow path, keep right and hug the right side of the cave to the exit point to the tunnel.
6. Run through the tunnels and stop at the steps.
7. Use your Bow to snipe the third Reaper that is at the top of the steps. Alternatively, just run up and melee it to death in a few hits if you're strong enough, just be sure to hug the right wall so that the phantom on the left can't hit you.
8. Once you kill it, run up and collect the loot, mostly Splinter and Chunk of Dark Moonstones.
9. You will reach a short passage and see the Black Phantom Dual Katana wielding Skeleton with its back facing you.
10. Equip the Thief's Ring and walk (don't run or it will hear you) till you are behind it.
11. Cast Soulsucker to kill it.
A good way to measure your step is as follows:
1. Before entering the passage, align your feet with entrance.
2. Walk in and lock onto the target as soon as you're close enough.
3. Walk your feet, once it crosses the exit. You can take at most 2 more steps.
If you are a bit further away from it, tap your left thumb stick slightly to inch closer. Try to remember the correct position so that you can repeat it again. If the Black Phantom turn its back, start running and remember to turn off the auto-lock. You can try again if you run far enough (take the right turn).
Video tutorial: 4-3 Farming souls - BP skeleton soulsuck