Okay ... so Tomo used a different spell (and a different chant) than I was expecting. Dad has been affected because if/when Jun goes down he has to fight his daughter. When facing Tomo, Gorou takes a 50% hit to ALL stats. Also, if we add in the 'I hate you' spell cast a few days ago those stats are further reduced by ~20% making him withing range of a normal NPC/field boss. Also also, Jun has also received a boost to his stats. Determination shot up by a good bit (maybe +10) and Focus also increased by about the same amount. Jun may also have some ancillary stat boost from Tomo's presence but we'll have to wait and see what those are ... for now his stats are fluctuating.
A victory by Tomo because Gorou can't be serious with his little girl will leave a bad taste. If anybody thinks that Tomo could really beat a serious Gorou (meaning he sees her as only an enemy and NOT his daughter) I would say you are deluding yourself. He can't be serious with her right now and he would surely pull his punches because he doesn't want to make her more angry at him/hate him more. He is effectively nerfed against Tomo. That in and of itself makes a Tomo victory against Gorou less satisfying.
I do have to say that Tomo actually used her brain and came up with a better solution than what was suggested at the end of the previous chapter. Now we see Jun motivated to give a more serious effort. And she only jumps in when he gives up/can't move anymore. I wait to see if this actually goes anywhere for Jun's fight against the final boss.