In terms of magic, there were three ways to deal with elemental energies like the one used by the first arsonist and the recent intruder.
The first was to use overwhelming force to overpower it. The second was to be better at manipulating the specific elemental energy.
Unfortunately, while Mandra was skilled at all kinds of magic, she was not adept at the unorthodox way the intruder was using his fire energy. It was close to but not quite magic. It moved like an extension of his body.
If she had her peak strength, she could have taken control of it. But with her limited pool of strength right now, she had to rely on the third method. She had to use elements that naturally suppressed the fire.
Water, ice, and wind if used correctly were all good against it.