Becky had two AoEs in her repertoire; Domain of Blue Fire and Cold Haze. She could only execute them when there were no allies around in the spell's ranger.
That's why Becky wanted Eren to prioritize her AoE execution before any of her teammates started with their moves. She couldn't control her AoEs the way she wanted to. She couldn't make her spell exclude her teammates and only affect the team's opponents.
So instead of trying to control what she couldn't, Becky wanted the team to start with her spell. In a way, her thinking was not wrong.
She wanted to cast her AoEs with peace of mind. Then she would retreat from the frontline and let her teammates take care of the remaining targets that survived her spell.
But the butcher was thinking long term. He had asked her to learn to control her AoEs. With her AoEs, it was possible. She was just lacking the motivation to do so.
So Eren gave her just that. He started suppressing her AoE executions under the pretext that his teammates' battle styles weren't able to accommodate her AoE executions.
He also gave a reason that her spells would be used as a backup plan if and when things start turning sour for the team. All the members of the team would retreat to the backline during crunch time and her AoEs would be used for Eren and the rest to catch a breather mid-battle.
As Eren demonstrated to Becky, they both can play the game of making battle strategies tailored to their tastes without caring about others. He came up with a plan that was as right as hers without treating her AoEs as opening salvos.
Meanwhile, Eren also told Becky that if she could optimally control her AoEs so that they won't harm the teammates, then she could cast them almost anytime she saw fit. With this subtle suppression, he had given her the motivation she needed to work on her control.
Now she just needed to act on that motivation. But instead of getting riled up, Becky procrastinated. The procrastination led to dissatisfaction, which in turn became the reason behind her sour mood every time Eren said something.
Eren eventually figured out the reason behind why Becky was trying to pick a fight with him. But he just smiled in his head and carried on doing what he had planned, ignoring the teen's tantrums.
So when the chance to showcase her prowess was given to her, Becky decided to let it all out and release all her frustration.
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"Blue Fire Domain!"
Becky executed her AoE as soon as the beast group got into the spell's range. The ground beneath her had gotten covered in sky blue flames which started spreading in all directions. They covered the position the beasts were in while running. And only then did the flames stop spreading.
When the flames touched the boars' hooves, they began to climb on them as if they were searching for food. Preys that radiated body heat.
The flames didn't burn the beasts' skin. But the beasts felt like their hibernation period had suddenly sneaked upon them. Their bodies' temperatures dropped at an alarming rate.
As a result, the beasts couldn't help but slow down their advance. The sky blue flames started showing a darker shade of blue as they absorbed the body heat of the beasts. The flames seemed even hungrier and started to absorb more of what they craved. As if these flames were intoxicated by the warmth of their targets.
Almost no beast could fight off the sudden lethargy felt by them because of the blue flames. They stopped in their tracks as their bodies started turning stiff because of the thin layer of ice formed over their skin.
The spell did not cast ice. It was the surrounding moisture getting condensed on the beast's bodies that had eventually turned into the thin layer of ice after whatever minuscule heat was drained from it.
Becky felt glad that her AoE was working fine mid-battle just the way she wanted to. She had subconsciously feared that whatever bare minimum control she had over her spell would have been lost because of Eren's suppression of not letting her use it during their ranking battles.
But soon she got tense after seeing that two beasts from the group were still approaching her. Becky was sent alone in this direction. She didn't have her usual tank protection acting as a net to fall back on. She knew Eren was trying to push her over the edge with a formation like this.
Haash! Becky sighed and made a fist of determination with her hands. If Eren wanted to force the best mage dormant in her, she would just show him that what she had right now was still enough to make him listen to her.
Becky maintained the Blue Fire Domain's runic circle with her right hand to keep the spell active. And with her left hand, she started drawing another runic circle in the void.
"Cold Haze!"
Becky executed another of her AoE. She had practised hard at stacking her AoEs. it wasn't easy to stack spells in the F-Rank. And it became even more difficult to combine something like two AoEs. These few months of rigorous training were fruitful for her.
As a result of her spell stacking, Becky started sweating lightly. She had the wild thought of cooling herself down with her flames too. But too bad, her body as a source of the flames was immune to the spell's effect. The school of spell had been designed as such after all.
The effects of Cold Haze paired up well with the Blue Fire Domain. Instead of using the flames as a medium to absorb the heat, this AoE would use air to absorb the warmth from the surroundings.
The two beasts that were slowly but surely approaching the mage felt that their lungs had been filled with ice. The mana layer that had protected their bodies from external harm was already weakened by Becky's first spell. So when she executed her second AoE, the boars didn't have a way to fight back.
The two boars in the lead along with their mates behind them froze to death when Becky was done with them. Notably, the boars in the lead were only a few steps away from their now-panting target when they lost their lives.
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AN: VEH has 300+ Golden Tickets this month and counting. Thank you Greeko_suave, Venom30, Cardunkos, and all other GT contributors. VEH will have another bonus chapter in a few hours.
Non-canon Trivia: Someone like Becky would be one of the most useless superhumans in a zombie apocalypse, Lol.