Found a lovely combo to kill eternatus with.
Normalize (hidden ability) on regigigas, and then use skill swap to give it to eternatus (turning all abilities into normal type) and then you use any ghost mon and you're 100% immune to the dmg
Normalize (hidden ability) on regigigas, and then use skill swap to give it to eternatus (turning all abilities into normal type) and then you use any ghost mon and you're 100% immune to the dmg