Prove Nergigante could not survive that. That's all you need do. Unfortunately, every single outcome of Valstrax hitting another monster with an Ambush has shown them to survive.

Nergi has never had to deal with that type of impact from any monster what makes you think he won't be damagde by a force of something slamming into him at MACH 1 speed?

Also you can't use gameplay as proof.

Nergigante was shown going after Zorah, Xeno Oltura and Shara.

Nergi goes after Zorah cause he was an OLD DYING elder dragon and wanted the left over bio energy after he dies, Oltura isn't even canon and he couldn't do anything against oltura since he instantly got slapped away by its weird giant tentacle, and that was RUNIER nergi that went after shara and even then he only defeated them cause the hunter did 99% of the work for him.

Magnamalo, Teostra and Kushala were shown going after Allmother.

All of them instantly got turned into the hunters puppet after a single attack from allmother and even then allmothers power far surpasses theirs, not much they could've done against her.