When Gooper Blooper puts his mouth (or whatever it is) underwater and turns purple, it means he'll try to poison you during his next turn. So here's what to do: before he shoots you, use a sponge or powder puff (I think that's what they're called). When Gooper Blooper tries to poison you, the sponge/powder puff will absorb the poison and shoot it at Gooper Blooper, giving you an advantage.