How do I get past this?

So I'm trying to beat the boss at Giant Step in earthbound (SNES), and when I use the offensive PSI attack, (The one named after your favorite thing, I named it PSI Weezer), It does a lot of damage and kills his backup. Issue is right after he bites me and i lose 70 HP (My max hp is currently 90), and no matter what I do from that point on, he bites me again and I die. If I can't be helped past this, I will become genuinely convinced that this bossfight is impossible.
I'm playing the game on NSO if that means anything.
EDIT: Right after posting this, I did another attempt starting with a different attack, and him and his backup just kept doing PSI Shield, so i was able to win.