After a lot of training and thinking, you settle on a stop-gap solution for the time being. Saving a few cards for later could prove fruitful should you need a specific skill or ability to counter the enemy.
Firstly, the matter of learning to use a gun. No character you can think of fits the bill better than Kusanagi Mokoto, aka the Major. Not only is she highly trained in urban combat and the use of weaponry, but she's also an expert on hacking and counter-terrorism operations. You can't think of a woman with a better skillset than that.
For the next two, you decide to play it safe and fill out the rest of your High School DxD team by picking up the last two of the original four girls, Koneko and Asia. Koneko is a good fighter, but Asia's ability to heal injuries using her sacred gear is invaluable. You won't have to stay out of the fight long with her around to help. You return to the lab and find Mitsuru tinkering with the Sledgehammer.
"I got my picks. Mokoto Kusanagi, Koneko, and Asia."
She adjusts her glasses, "Mokoto? I didn't think of her."
"Hopefully she doesn't need to charge…"
"No. She does not." Ghost in the Shell is something that Mitsuru actually likes even to this day, unlike her sour on and off relationship with High School DxD. She puts down her tools and hustles over to the computer, entering three blanks cards into the press and presenting you with the faces of three new heroines.
The blonde nun Asia Argento, the deadpan Koneko Toujou, and Major Motoko Kusanagi.
"Now that you're done, we can hand out of our new communication devices to the girls. I just finished tweaking the Sledgehammer." She returns to the workbench and places it into your open palms, "We can summon four people at once."
You draw your deck of cards, inserting Rias, Akeno, Lala and Chun-Li into the chamber. A second later all four girls are stood in front of you. While the other three are fully clothed, Rias is wearing nothing but a towel and is covered in water having clearly just stepped out of the shower. She doesn't seem to perturbed about her public nudity, instead running over and pulling you into a kiss.
"Hello husband."
"Hi Rias, sorry about the sudden call but…"
Mitsuru clears her throat, "Before you all leap on top of my friend there, I want to give you these." She hands out one of the phones to each girl, "To prevent any further mishaps – I've given these phones the capability to communicate between worlds. We can message each other, and you can set yourself as available to summon at any time."
"I'm always available for our cute husband," Akeno giggles.
"While that's very romantic, you're actually not," Mitsuru grunts, "Rias just stepped out of the shower. I can imagine several situations where you would prefer to not be interrupted. While time dilation for short trips makes it quick, people stood next to you might see you change position or appearance suddenly."
"Wow, this is super amazing!" Lala gushes, "As I expected from Mitsuru-senpai!"
"No monsters to fight today, thankfully," you explain, "We can send you back as soon as you like."
Rias laughs, "While I'd love to stay, I'm afraid I don't have any clothes on hand." She snaps her fingers, eliciting a purple flame. Unlike in the series, her clothes do not appear out of thin air. Perhaps being disconnected to her original universe prevents her from using the spell correctly.
"I also have an engagement to attend," Akeno explains sadly.
"Same," Chun nods.
"I can stay for a while," Lala smiles. You unlock the chamber and withdraw Lala's card, leaving the remaining three inside.
You give Akeno and Chun a kiss to make up for it, "Alright, I'll see you girls later."
"Bye!" the three wave in unison, before dissolving back into their own worlds.
"Now I get hubby all to myself!" Lala declares, hugging your left arm and nuzzling her cheek into you.
"Nice to see you too, Lala."
She peers around you and spots the new cards in your hand, "Oh? New girls to join the fun!"
You blush, "Yeah. Two of them are friends of Rias, and Motoko is a special forces officer. Serious kind of woman." You aren't sure if she'll get along well with a girl like Lala. Even though she's a powerful alien princess with a genius mind, she's also a naïve high school girl filled with energy.
"Our little family is getting bigger!"
"…We're already pretty big. Chun told me that I have something crazy like five-hundred wives in her version of this story."
Her eyes light up, "Wow! Will he have enough time to write chapters for all of them?"
"Huh?"
Lala ignores your pointed question and hugs Mitsuru from behind, "What are you working on boss?"
"I was improving some of our equipment, but now that I'm done, nothing."
Lala looks around the garage for a moment, "Is there anything you'd like to build? I brought some extra rooms." Lala pulls several small devices out of her pocket and holds them out to Mitsuru, the same devices she used to expand your apartment.
"I may have an idea or two for how to use these."
-------------------
Mitsuru and Lala had run off to do whatever they were planning, and that was when you noticed that the mini-fridge you kept in the garage was running dangerously low on unhealthy drinks. You grabbed some cash and decided to walk down to the nearby store to get some while you have nothing better to do.
The route happens to take you past the location of the fight you had yesterday. As you approach, you notice a fairly large gathering of people gawking at the damage and taking pictures. The road is torn to shreds and several of the surrounding buildings have shattered windows and broken brick walls. You really did a number on it; you need to learn how to contain the damage better in future.
The area itself is under heavy construction, with workers removing the damage road surface and sweeping up the shattered glass. The strange thing is the shrine that's been built against the temporary railing. There are several images of you in your stupid hero outfit ripped from Mitsuru's promotional campaign. Dozens and dozens of signs and sticky notes declare their support for you or gush about how cool it is that there's a 'real life' superhero.
Almost makes you tear up a little.
With that said, Mitsuru is taking a big risk by making this public. She's a genius, she probably can get away with all of her plans without getting touched, but you never know – a lot of people are about to be very interested in this technology you're using. What's to say that some crazy government official isn't already trying to reverse engineer it and make an evil version of you?
That'd be cool.
On your return trip from the shop, even more people are waiting there. Are they expecting you to show up and sign autographs or something? You shake your head and push past the crowd, with little intent of stopping and speaking with any of them, until…
Bump.
You back away from the stranger. Did he seriously just walk right into you? "Oh, sorry pal," he smiles, "Wasn't paying attention."
"No problem."
"Everyone's getting really excited about this superhero stuff, huh? Can't move for them."
You stare a hole through the man standing before you, blocking the one open path past the large group of spectators. He pats his own chest boastfully, "Can I let you in on a little secret? They're looking for me."
"Uh-huh."
"Yep. Came down here and beat those baddies good, didn't I?"
You step to his left and successfully evade his grip, "Listen, you're gonna' have to find someone more gullible to pull this scam on."
"It's not a scam dude!"
You walk away but he follows you down the street, "Why the hell are you even telling me this? Someone's going to get the wrong idea and drag you to the cops."
"Why would they do that?"
You stop and turn back on him. "I don't know, millions of dollars of property damage? Vigilante justice? Illegal possession of a firearm?" Each successive offense causes his shoulders to dip lower and lower until he's practically cowering underneath you.
"Okay, maybe you're right."
You shake your head and finally manage to get away from the strange man. People are willing to do anything to become famous these days. What's the point? You keep your face hidden when you fight anyway! Unbeknownst to you, an observer had witnessed the entire thing.
"A fake, huh? That's a pretty nice idea!"