Whose and When did Clare felt the aura of yoma as strong and ominious as of Riful for the first time?
While it's true that normal human can't sense youki aura, those with sharp sixth sense can, So it's not that human can't feel their aura, but it's just hard to do. From Claymore 86, The Touch's Sin.



And it's true that you said she met 3 Awakened Beings but none of them were on Abyssal One caliber. Now, you mentioned that Clare might not be talking about the awakened being rather one of Claymore warrior, while this is probably true but considering what Galatea said right after that, they must be talking about the enemy rather than their fellow warrior.



Now this left us with only one possible choice, Priscilla. But did Clare really feel her aura? We don't know for sure about that. However, she saw Priscilla killing Teresa, and when she faced her at close distance, she was overcame by fear, trembled by her presence. From Claymore 24, Marked For Death 7



Now that Clare can feel their yoma, she must have remembered that feeling and realize how strong Priscilla's aura actually was.