I would say Erza Scarlet (Fairy Tail) would probably win in a one to one fight because her power set is more designed for combat whilst a lot of abilities that Rias Gremory (DxD) possess require her to work with the team for example
Rias Gremory (DxD) ability Extinguished Star is extremely powerful but it requires Issei to activate it and Healing which cannot be used in combat and requires her embracing the injured person and both parties to be completely nude with skin to skin contact (perhaps she should work as a nurse)
the only real difficulty with a fight between the two of them would be Erza Scarlet (Fairy Tail) getting close enough to attack this is because Rias Gremory (DxD) has a lot of raw power as well as two abilities that would assist her Power of Destruction and Defense but if Erza Scarlet (Fairy Tail) moves fast enough and continually dodges to drain Rias’s power she would eventually win
I would say Erza Scarlet (Fairy Tail) would probably win in a one to one fight because her power set is more designed for combat whilst a lot of abilities that Rias Gremory (DxD) possess require her to work with the team for example
Rias Gremory (DxD) ability Extinguished Star is extremely powerful but it requires Issei to activate it and Healing which cannot be used in combat and requires her embracing the injured person and both parties to be completely nude with skin to skin contact (perhaps she should work as a nurse)
the only real difficulty with a fight between the two of them would be Erza Scarlet (Fairy Tail) getting close enough to attack this is because Rias Gremory (DxD) has a lot of raw power as well as two abilities that would assist her Power of Destruction and Defense but if Erza Scarlet (Fairy Tail) moves fast enough and continually dodges to drain Rias’s power she would eventually win