Use a fiery slash.

I haven't used a spell since Mend, and I doubt that uses up much of my mana or whatever. I'll just cut the damn lock off.

You slide your hand down the flat end of your sword and whisper, "Flare Blade". Trailing behind your movements, the metal erupts into a bright flame. You then swing it to the side, and squeeze the handle tight of your new fiery short sword. You know it doesn't stay for long, so you position yourself beside the cage door, and swing with all your might against the bar of the padlock.

Sparks fly and metal meets metal. The intense heat of the weapon overpowers the weak, old iron, and the lock crashes to the ground. You pull the door open, and the captives make their way out as the fire slowly dissipates.

"Pretty impressive," the girl finally speaks.

"I guess," says the boy. "But uh, thanks for coming. I don't know what they had planned for us."

It seems like back in combat, everyone has joined in. Even Eva is throwing out a couple spells, and Nyllia's knights are backing her up.