Many forms
Since Sigma had mana to spare, the sand beasts weren't a problem, even though he needed two arrows to deal with them. The only problem was that they were annoyingly insistent… they came toward them all the time. In the middle of the day, in the middle of the night. When they were trying to sleep or trying to eat. At all times, at least two of them had to stay on guard to face them.
That wasn't a problem per se. The real problem was the fact that Sigma and Lucy could only kill the beasts while being a bit noisy, so the nights started to become quite difficult for the group.
"It is annoying since my elemental arrows can't effectively use their effects on them… freezing and burning them isn't possible since their sand body is almost intangible," Sigma thought.