Water is the safest choice. If she use lava, she has to do both storage magic to bring out the lava AND space magic to get the hell out of there before she gets roasted. But that's impossible at the current kumo because:
1. she has to maintain the storage magic to pour the lava. If she stops, the puppets just have to destroy the earth blockade to get the hell out of there.
2. She might not be able to cast space magic if her concentration is disrupted especially since
3. There's no magic in charge 1 & 2 so she cannot do multitasking as better as she was before.
4. Fire is taratect's natural enemy. Remember in the earlier chapter, even the greatest of taratect species, Demon King Ariel, the origin of Taratect, only has flame resistance lv 5, while she has nullification or super resistance against other elements. Even though Kumoko has immortality, it's still too risky.