The lava won't be so easily stopped (it will slow down, harden and fall into smaller blobs, but the Naga can and will make more). Fires are extinguished well enough.

However, if Nia distracts her enough with attacking the lava, Ruby has a good chance of attacking her without being interrupted. Any wound will bleed poison instead of blood.

Should Ruby aim and destroy for even one of the emeralds, the Naga will noticeably slow down.