Download the zip, rename it to exe and run it as admin. Make sure to turn off your antivirus.
Just kidding. Backup your saves and extract the files from the archive into your save folder.

https://drive.google.com/file/d/1CBxqcvdxa0Uuh3udgkpafp7LgrF9oNun/view?usp=sharing

%appdata%/Qute/NatsukiChronicles/

For Seventia! is the only difficult achievement because of Advanced Training on extreme where you have preset weapons and no ex shields. The other stages can be finished by grinding high level weapons and ex shields.

Slot 1 has 500k cash to play around with, but all three of them should award the 500k achievement, since it counts total credits earned. Play a stage if it doesn't pop right away, in case the achievement gets triggered upon updating your money.
The other slots are copies of slot 1 with all but one weapon already purchased.

---

For Seventia!
I left one stage unfinished in all three saves. Make sure you don't accidentally finish it, otherwise you also get the all stages achievement....

...and that would be cheating!