How do I do it~~

You can buy them by clicking the weapon icon in the bottom left corner of the screen. If you combine similar weapons, you can create a higher-tier weapon.

Slurp! Looks like you're having trouble finding the shop, heh. You can usually check the summon tab at the bottom of the screen, but it might only unlock after clearing a specific stage. I'll look into the exact location more thoroughly myself, so please wait a moment, hmm!

For diamonds, buy skins instead. It's more efficient. Buy all the gun skins after you get those.

If you just want to clear the hidden stages, I think the reincarnation skin weapon order is the fastest.

If you equip A-rank weapons and open the skins first, you can clear it easily.