As for your question I couldn't really tell, by this I mean that since a while the game give a damage booster to event card, making the "main one" often the worst opponent for the boss and thus resulting to be able to easily beat it. That pretty much why I wait for final quest to be realease since it present a far better challenge than all previous one.
We don't really get the opponents stats, just what they can do at best, for example Godzilla (Gojira ?) can inflict "burn" which is troublesome if the fight drag on, but like I said earlier with the event Card; Kanade is of course immune to it.
I would probably be able to tell you more when the final quest is out, to see how strong it is at its max level.
First, in the end that Gojira wasn't much of a threat, he did have an extra in the fact it was immune to the 10 first attack it received and was doing some pretty decent damages. Well my party was not very fair too since I did used my best Hibiki and Chris cards. A very strong Boss in its own right but still not the worst I ever faced, Berserk Hibiki and Maria gave me a harder challenge... I think.
The event got a story yes, I can't be totally sure but from what I saw. Our Symphogear Users got caught up in a fight against Gojira, join a special a group to fight it, the "corruption" linked to Gjallarhorn must be the cause of the mini Gojira as well as why the Symphogear Users are able get their special gear once in contact with the same "corruption"... That's pretty much the git of it from what I understood myself. Unless you want a full version.
Unfortunatley I never thought about searching such a thing, so I can just tell to try your chance.
Saying that Gojiro is weaker than Berserk Maria and Hibiki wouldn't be fair to him, I was rather speaking about "Final Stage - Max Level", For Maria and Hibiki, it was fighting them alone; Maria in the first round and Hibiki in the second, the both of them were also of two different attribute and could give some troublesome ailment. In the other hand with Gojira; the first round was fighting a Karma Noise, 3 Gojira's minion in the second and itself in the last, all of them the same attribute which was the worst for him to have against me (contrary to Hibiki's and Maria's)
AS for the minions, the game put them on a level similar to a Karma Noise if maxed I wuld say.