Assuming for the moment, we continue trying to win without Homura bailing us out...
I think a dodge attempt is pretty much null vs the million gun salute. That means shift to full blocking defense.
Currently we have 45 (sword skill) + 30 (armor) + 20 (kite shield). We can bump the kite shield to a great shield for another +20, and add the helmet for +5. That would give us a grand total of +120 defense, vs Mami's +90. That would give us a 75% chance to block (66% if we add in an Offensive Boost), perhaps more if we can continue destabilizing her.
Multilayer shield charge? Make the biggest shield we can manage, then put several layers of platforms in front to dampen the shots?
Decent idea, but dunno if we'd have time to get the platforms up in addition to the shield change and the helmet. Kyouko may toss up a barrier to help us, though, which would be another small edge.
One thing from the update, which might just be flavor text because of the imbalanced roll, is that we jumped back after stabbing Mami with the sword. If we stab with the sword, we need to
stay right up in her face.
If we go full defense first (ie: defend against the gun barrage, then attack), we don't have to worry as much about the sword; we can change to the greatsword when we attack. The other option is to go full defense, but bring out the big sword at the same time and charge. That would continue pushing our momentum, rather than letting Mami get the momentum back, but could potentially add extra risk to the defensive side of things (or maybe make defense better, since the shot angle would be more difficult and we'd no longer be sitting still waiting to get hit).
One other trick that we pulled on Sora way back when was to add blades to the shield and charge into her.
And we still have Kyouko making her own plays.
Assuming aggressive, to keep momentum against the distracted Mumi...
[x] Upgrade kite shield to great shield, sword to great sword, and add the helmet.
[x] Kyouko to continue doing anything to constrain her movement; barriers galore. Or, take advantage of Sayaka being the distraction now.
[x] Charge at the fake-Mami. Manifest blades on the shield and ram with a shield bash.
-[x] Use a simultaneous strike with the sword to cut off avenues of escape.
Not sure whether to add the offensive boost or not.