You gain Shards by killing enemies and Magic Extract by extracting it from prisoners.
Shards are split into stat-specific shards that boost that specific stat, while Magic Extract raise every stat by nearly the same amount.
These 2 items are very easy to obtain on the "Double Fake" stage with the 100 Asagis.
What you need:
Equip the Megaphone in the Second Weapon Slot of your Sage and head into the "Double Fake" Stage.
There you try to kill every Asagi with Land Decimator. If you can't, try turning down the Difficulty in the Cheat Shop.
You will gain a lot of Shards from this, and the Megaphone captures a lot of Asagi-Prisoners too.
After beating the stage a few times, head back to your HQ. Heal your MP at the Hospital and Interrogate your Prisoners/Asagis. Threaten works wonders there. Then extract them all into a bottle of Magic Extract.
You can combine Magic Extract and Shards in the Strategy Assembly. If being used during battle, these will raise your stats by the amount that the item has. I highly suggest using a Maid with the "Licensed Caregiver" Evility. This will make Items into Area-Of-Effect Items, which means you can give Magic Extracts or Shards to multiple Characters at once.
If possible, raise the Difficulty in the Cheat Shop. This will make farming way easier, but remember to not set it too high, because you still need to kill every Asagi on the map with Land Decimator.
If I remember correctly: The max. amount of stats that can be gained through Shards AND Magic Extracts should be 10 Million.