The line "seven hells" is an allusion to Dante's Inferno. In it, Hell has 7 levels, each corresponding to the seven deadly sins. Then the saying "seven hells" irl is a colloquial term used to reference Dante's Inferno. So Athen saying it is just her cursing, most likely out of frustration with the situation fighting you has presented.

That being said, the 4 elemental fiends are also demons in Inferno.

And Kokytos boss of 9th circle also comes from Inferno, but is written as Cocytus which is the frozen lake found below the 7 levels of Hell where the devil and nonbelievers are frozen in despair. Cocytus means "lament". And if we do a little math, knowing that purgatory comes before you enter hell, plus the 7 levels, and then the frozen lake would also technically be on the 9th floor or circle if you will.