"Walker! That was the last boulder!" The fourth rock ice wall had really strained Walker. After the second he had to get a mana potion. Mostly because the earth and water mages were struggling. It wasn't so much the mana usage overall, but the sudden mana usage paired with the higher mental burden.
This had not been what some of the earth and water mages were used to. Therefore, it led to the mages expending more mana than they actually needed. That was just something they would have to train over time and something that would change as they became closer to their elemental choices in training. They had to understand the earth and water better to minimize mana cost overall.