The Corrupted Volcano was a vast area, no smaller than the Corrosive Lakes, and possibly even larger if you took into account all the areas reached by the lava rivers. Many of them originated from the volcano, but a significant portion came from the ground.
There were several Twilight Crystals in this area. This meant at least two things: there was an underground level where the Twilight Crystals were hidden, and there were several powerful monsters that prevented others from absorbing the crystals before their time.
For the two Phantoms who came to the Corrupted Volcano on a mission, these were favorable conditions. They didn't need to rush. They could explore the area, find the Twilight Crystals, and absorb them after defeating the strongest monsters.
However... When there were two or more Phantoms on a mission, they needed to work together as a team.