I just got this book and I wondered to myself, if I take hits with this on but don’t lose HP will I still get a Perfect?
So I go to the first battle in the game, kill all but the last 6 enemies and let them march towards me. They all hit the PSI Shield and then… nothing happens. The battle just goes on. So it seems like if the very last enemy just rams into your PSI Shield, the battle will never be won.
But while I’m here, CAN I get a Perfect if the PSI Shield absorbs the blow?
We’ll try to fix this for sure.
And as for your question, YES, that’s the entire point of PSI shield.
PSI shield makes it easier to get a perfect, but harder to win the battle. Much like pushback and crystal are “clutch” spells that you can use to trade PSI for “don’t screw my perfect” at the last minute, PSI shield basically gives you a way to directly trade PSI for “don’t screw my perfect.”
It comes at a pretty high PSI cost though, so in real battle situations it’s generally advisable to spend that PSI on absolutely anything else you can rather than have an enemy walk straight into Azra. If you’re not careful with it, you’ll preserve your perfect but screw yourself out of PSI to the point that you suddenly just lose altogether. At least, that’s the vision for this book 
We’ll see if it works that way in practice.