I get that Eva generally did not have to use the technique as she is already a monstrously powerful vampire. However, in her final "fight" with Negi at the end of Negima, it appears that her Magia Erebea was just as powerful as Negi's.
In what sense was it considered incomplete?
Okay from what I understand so far about the whole Magia Erebea is that it can be a way to become immortal. Now within the manga, UQ Holder, we find out that there are varying degrees of immortality and what is suggested that they are not "Complete" because they still need to eat, they can become injured and, to some degree, die.
Spoilers
Negi, on the other hand, uses a Complete form of Magia Erebea, suggesting that he is a complete immortal, having no need to eat, cannot be injured and cannot die in any event and will continue to always exist.
Now I'm assuming that everyone knows how we eat is a necessity because that allows our bodies energy to do things.
How Magia Erebea could potentially, and this is only in theory, make someone immortal is to continually "flow life, energy or magic" into the user. Now as explained in Negima!, Magia Erebea is a techniques that allows the user to absorb and take in magic. Now it is possible that the user of Magia Erebea can use the magic to sustain their life and the complete version could do so passively without the need of incantation or commands.
Now looking at Eva, she still needs to eat and feed, like all vampires, on blood or more accurately, explained in Negima!, that she feeds on a person's mana. She can still be injured and there is a possibility that she can still die when the time arises. Due to this, it can be said that the Magia Erebea that Eva wields is incomplete because the Magia Erebea has not granted her "Complete" immortality, also Eva still needs to activate it through commands meaning that it's not passive.
I hope that helps but you probably know all of that by now...
