"Polluted Water" 

The mermaid asked her teammates to start the counterattack and stopped being passive. 

They could be passive before, but after that monster came, they had to fight back. 

The corrosive liquid attack was just too lethal!

After the mermaid told the group to target the corrosive monster, the sonic guy immediately changed his target and used his sonic attack to hit the body of the pitch-black monster with some strange liquid dripping out of its skin. 

Many other monsters created a 'vacuum zone' a few meters around this monster simply because the other monsters didn't want the strange monster to corrode them to death with that vicious black liquid from the skin of the monster. 

The sonic guy's attack was not quite visible but the surrounding water rippled before hitting the targeted monster with a burst. 

There was some kind of small 'explosion' in the water and the corrosive monster was surely hit by the sound wave attack.