This trap would most probably work for the guys that Roy had fought against until now. Especially if he used Alma's help like now, that she was actually trying to freeze the guy on top of them.
Alma's ice, and more exactly her ice cover was extremely bendy in such cases as she was able to actually trap stronger experts, or at least delay their movements.
In fact, her Ice seemed to even have a special ability or effect that would act directly on the target's soul, as even Roy could feel its traces inside himself.
This strategy or formation even if it didn't guarantee the target's death, it would at least guarantee its heavy injury, which would make its death close to imminent later.
But the target, or the opponent this time was nowhere close to being just a Rank 4 Master, as he was actually a Rank 5 one.
