Klaus had already predicted Alice would help at the crucial moment, to make sure it was possible, he took the liberty of also attacking her opponent after getting help from Void. He did not need to communicate his intentions to Alice, after being friends for so long, she already knew what was going through his head when he helped her out.
Her opponent was an Eighth stage Sovereign, not as powerful as the Ninth stage Sovereign Gnome chasing after Klaus. As long as they could deal with Klaus's opponent, her opponent was as good as dead.
Being enveloped by Klaus's heavy water, the pressure alone almost crushed the Gnome. The Gnome survived the initial pressure, only to realize it would have been better if he did not resist the pressure and let it kill him from the start: at least that way he would not have to go through the struggle of being unable to breathe.
The water passed through his nostrils, mouth, ears, and even his eyes, drowning him in a matter of mere seconds.
