Yeah… If Erza is a problem, as said by Sharkai, you can replace her by Laxus who is in her tier and even Gildarts, slightly above but not so far.
The gap between Eileen and classic Erza was so high, that I think even Gildarts can't compete with Eileen. Eileen was too strong, even for max PoF Erza where she was far above Laxus' Tier (like Dragon Force Natsu is far above Gajeel's Tier for exemple).
But if PoF Amped is also a problem, stop speaking about Erza, and Fairy Tail's characters.
See God Serena and Eileen against the same foe to make an analogy : Acnologia.
No offense for God Serena, strong Spriggan, but when Acno arrived, he was ready to fight but died in less than a second. Acno did only blitz him and stroke him to death by a slap.
Eileen did far more against him. She surprised him by her strength. He asked her her name. He was curious. Agree, Acno was playing but Eileen did the same. She only took mesure of his Legend and knew that he was too strong for her. Man, she did better in their duel than the 7 Dragon Slayers individually.
This analogy showed us the gap between Spriggans Tier like Serena, too strong for classic wizard and Eileen/August's Tier, too strong for the Spriggan's Tier. And finally Acnologia, too strong for August/Eileen's Tier. It is pyramidal