When Arad's expansion shattered, a sea of water splashed down. Arad wasn't trying to win by using his expansion, that was nothing more than a distraction, he was the bait, giving Undine enough time to build up water outside the expansion so Gamond wouldn't interrupt her.
Undine wasn't just creating water, she was putting her life and name on the line, going far over her limit and exhausting herself to the bones to make sure she fills the entire maze of possible. She would be a world power capable of destroying the world by flooding it whole, and this is what she did here.
The multi-dimensional maze of Yog almost flooded, filling with several oceans worth of water and putting Gamond beneath enough pressure to make the depth of the ocean look like a joke.
Gamond covered her body in ice to withstand the crushing pressure while Arad's body was crushed into a tiny lump of flesh. "Undine! You just killed him! Is this how much you want to kill me?"
