Hi im from the future. I comment this when the waterfall update come out. If you have undyne USE IT! Undyne spear shield power will not break until got attack by range or magic attacks and GASTER only use physical attacks. That means use spear shield from undyne then your invisible from GASTER (dont try this his health are super long)
with papyrus and toriel you can EASLY kill gaster but HELLA EASLY , gaster cant destroy your bone wall so you become immortal and then toriel with her immense damage destroyes gaster
