In order to defeat galacticmon, you need a strong digimon of any mega form, with at least 5000hp to ensure a couple of turns for attacking. The first part of him is the tail where its all about physical attacks. If your digimon has Counter Crest equipped, he will counter the tail's attack back to it if your digimon gets hit, and the counter's attack is equal to the physical damage you recieved from the tail's attack + any bonus damage gained from (Bolt or Fire or Water or Dark or Ice) Power (1 or 2 or S) + critical hit which doubled the amount of damage inflicted.