On Test Server, Fluffey said that they added proficiency progress in the last "boss" of the soulpit:

However, soulpit creatures are based on bestiary, so considering the 2800% proficiency progress, you can get by completing it:
Harmless: 28x1 = 28 Exp
Trivial: 28x30 = 840 Exp
Easy: 28x70 = 1.960 Exp
Medium: 28x100 = 2.800 Exp
Hard: 28x165 = 4.620 Exp
Challenging: 28x240 = 6.720 Exp
Also remember that this experience is only obtained while killing the last creature (the strongest one) and doesn't count in any of the others. It's not a good experience, but is better than nothing. If you still didn't completed the 200 Soul Cores, you can make a run and do all of them and get some experience on your weapon for it, so it's better doing soul cores now than before.