Kushina was down for the count, and the strange masked man had teleported elsewhere. While the Kyuubi was occupied, Minato had only one hope left.
He needed Sage Mode.
He freezes in place for literal seconds before the power kicks in (Seriously, this shit happens. Bullshit you suck as using Sage Mode. You are a fucking perfect sage, so you're better than Jiraiya) and he vanishes to the town with the Hirashin.
Minato is blown away by a wind blast from the Kyuubi, but recovers quickly. Blocking the force from his eyes with one hand, he throws one of his trademarked kunai at the beasts face. The wind causes it to lose momentum before impact, but it was enough.
Teleporting to the kunai, Minato rapidly prepares a Rasengan and smashes it into the nine-tails's head, blowing it backwards. The force doesn't quite knock it out, so he throws the kunai mid-air, teleports to it, grabs it, tosses it, and repeats until he slams yet another Rasengan into the thing.
Bam, boom, done.
Minato warps to the kunai above the masked strangers back, slamming the rasengan into his back and forcing his hand past it to lay a chakra marker. He presses off it to leap back before ripping his soul out with the Reaper Death Seal. Then, like someone who isn't an idiot, he just leaves with the soul permanently trapped within the Shinigami without finishing the technique and thereby saving his own soul for the remainder of his life.
This... This is what should have happened. No Kyuubi, no akatsuki, no Madara, no Kaguya, and best of all, Naruto gets a fucking family. This guy is a suicidal moron, and people don't truly understand the grasp of their powers.