Ok, I compiled the best awnsers I found into this:

Helpful tips
• You should be lvl 25+
• combining use of the powerful badge and the great badge would be helpful
• Bring a Falling Star if you have it ( I think it's a equitable special )
• Power/Great badge combo gives between a 1.5x or 2x attack

How to beat him
&Quote from Game FAQ,
To start off, just jump on him. Do not use special attacks as that is a waste of SP, as all attacks deal zero damage. When the mini Dark Stars come out, either use Falling Star on each, or use a different special. Either way, when the minis attack you, hold BOTH A and B and hammer the mini. If you need to, attack the bomb and mini at the same time. Afterwards, repeat with the second. Now, if you have it, use the gauge, and use Falling Star. Make the other bro use it, too. If that doesn't kill it, just rinse and repeat.

What I got from that
Step one: Jump on Him
Step two: Dodge attacks and ect
Step three: when Dark Stars come out, us approved special, falling star on each mini Dark Star
Step four: when min's attack, hold both A and B and hammer attack
Step five: if needed, attack bomb as well.
Step six: Repeat 1-6 as many times needed

Hope this helps! Got the info from Game FAQs! Check 'Em out!

This request was made by: chrisskywalker