Bossfight Guide:

When you approached the device resembling female sex organ (where fetus is developed) (the theory about Giygas being fetus was rejected by Shigesato Itoi btw) Devil Machine and the fight just began, have Poo using Brainshock Alpha to Heavily Armed Pokey, thus will make him feel a little strange. And after you got to Giygas' wounded state, use ALL of your best attacks (PSI Rockin won't work whatsoever) and use Brainshock Alpha too.

For note: Giygas and Heavily armed pokey are both vulnerable to Brainshock, thus it will make enemies attack themselves

While Giygas feels strange, DO NOT use any attacks towards him, it will make status aliment on Giygas gone.

Then just pray with Paula and use Defend on Ness, Jeff and Poo (or Do Nothing when one of party members are numb, additionally, use Ness for healing Paula since she is absolutely needed to defeat Giygas)

Also, i really recommend playing EarthBound™ if you enjoyed playing Undertale or other Mother-inspired games, you can also get on Nintendo Switch Online subscription or get SNES and then EarthBound cartridge as both legal ways.