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.
Once you defeat the tail, you will move to the body, the Ragnarok Cannon, whichwill attack you and reduce your HP to 1 and then skip to the head.
The head has 2 attacks. One is an unmeasurable physical attack named ????? which can be countered with Counter Crest if equipped. There is also a wind based attack, that does less damage than the ????? attack. The head can also Counter any attacks it evades from you, and has a fairly big amount of defense so it will often get the chance to counter your attack. After he is defeated you get an FMV of galacticmon exploding and falling. If you play the PAL version of the game you will be able to continue afterwards. Otherwise its the grant finalle and the game ends here.
Note: you cannot counter a counter