Given Homura's dialogue about Madoka's "weak spot,"
and since Homura is a time-traveler
then it stands to reason that the only way Homura would know this would be via going back in time enough times to have learned it, hence my adding the time_paradox tag...
This really was always your weak spot, huh...?Hya...!"Always"... Even though... this is my first time... Why...?It's a secretAh, ah...Hauu...!I can't~Homura-cha... nn!