Goolix will appear in The Impact Site in the 1/5 chance. when he appears, Take 50 Blue Pikmin or less and Aim for that strange round object inside the goolix that shows a yellow-Green-Black colour, control your Pikmin with the C-Stick to that ball, press Y to release your pikmin, and wait till the end, goolix will die.

Note: Don't Use any other colour Pikmin, cause goolix has a body of water! And make sure you know that Red and Yellow pikmin hate that element.