She couldn't create massive shields, nor was she able to use high-leveled magic.
Aria could also see that Ji-min wasn't able to efficiently defend while launching an attack. Thus, she was cornered by Reik into a position where she could only defend, defend, and continue defending.
Meanwhile, Reik was able to continue lighting his sword up because the sword itself was a special item. It boosted his fire aptitude and cut down his mana cost for using the flaming spells.
Not only was Ji-min not able to attack, her stamina was also not that great.
She was breathing heavily. Her moves were sluggish. Ji-min was mainly a support mage.
That did not excuse Ji-min's fatal weakness.
In the battle, they would be stuck inside the dungeon for long because of the frightening difficulty. If Ji-min acted like this inside and couldn't keep up with the rest of the subjugators, she would certainly become fodder.