What level should I reach before I take on Dark Bowser?
What Level to Grind to
Every level you get will make it easier, but lvl. 40 is when you get the final rank, and that is, in my opinion, the best time to do the final boss fight: not too hard, not too easy.
Where to Grind
The most late-game area always provides the enemies with the most exp, and they will become easier as you continue to grind.
How to Counter
Dark Bowser has many attacks, each with its own method of countering.
He will throw a huge rock into the air, and blast it with dark fire. Press Y to guard against the rocks that fall directly on you, and press X to punch the rocks in front of you at Dark Bowser
Dark Bowser will suspend you in a cage, and charge an attack. Punch the side of the cage with good timing to get it swinging, and it will break free and fall on Dark Bowser.
Dark Bowser will prepare to punch. Punch his attack when it happens, and he will curl up into a ball and jump on you. Guard in your shell, and when he bounces in front of you, punch him.
He will throw a large bundle of dark enemies at you. Punch the goombas, koopas, bob-ombs, and low-flying bullet bills, Guard for the thwomps, high-flying bullet bills, and the balled-up Dark Bowser
When Dark Bowser grows, punch him in the belly to force him to disgorge the Dark Star. Then, vacuum strongly to inhale him. Mario and Luigi then get to fight the Dark Star, a part of the boss fight with its own attacks.
The Dark Star will extend a spike from its top and start spinning it. If purple sparks (not bubbles) are flying from the spike, jump over it. After that, it will charge the spike and point it up toward Mario, or down at Luigi, and fire it at the specified bro. hit it back with your hammer.
The Dark Star will extend his arms into the ground beneath the bros' feet. When it sends sparks along his arms, hit the sparks with your hammers.
In order to attack the vulnerable part of the Dark Star, you need to knock out its legs. In order to knock out its legs you need to stop its vision by attacking its glasses. Focus first on the glasses, then on the legs, finally on the Dark Star itself, before Dark Bowser retrieves it.
For the final stage of the fight, after Bowser talks about a KO punch, the X button will appear when you need to press it. When it does, mash it to charge your punch.