As much as a fan of "Louise summons x" fics as I am, A Summon Goes Wrong: Mahou Gakusei Louise is an amazing spin on a well-used idea. In fact, it's almost more of a Negima fic than a Familiar of Zero fic. It explores the consequences of Louise's accidental step into the world of Negima, with her heartbreak of never being able to see her home again and her new position as Evangeline's discipline.
It promptly starts off with a satisfying, exciting Curb Stomp Battle
from Evangeline to the school, then shifts into Louise's new life as a Mahora student. Louise fits in well with the wacky class of 2-A, and her growing fondness of her classmates is very heartwarming. Another interesting twist is the exploration of Familiar of Zero's magic system in comparison to Negima's, and the interesting solution Eva devises to get around Louise's magic problems.
The action is fast-paced and the fights well thought out and almost on the level as the manga's epic battles. There are a few punctuation problems and maybe the writing isn't as clean as it could be, but it's in my opinion one of the top Familiar of Zero fics on this page.