PLEASE tell me how to beat Bowser in the "Palace of Shadow". (Grodus is..

This is easier than it looks. I'd first tell you how to save your HP when fighting Grodus, but seeing as you leveled up during that fight (unlike me, and I still beat him in one try), it doesn't matter. The BIG thing to remember for the second fight is to take out Kammy first. At all costs. Her spells can heal either of them and she can mega-boost Bowser's AP. She has 50 HP, while Bowser has 70.

I suggest possibly Ultra rank Vivian to use shade fist, Or Flurrie's body slam would work. As for Mario, use Spring Jump. It costs 3 (two if you have the FP saver) FP to use, and if executed correctly, it does a lot (possibly even 15) damage to the target.

Or, you could just do what everyone else suggests and use that stupid bomb and go "Bomb-blast-thingamabummy," Even though that wastes major FP when you use it (around 9).
If you've beaten Kammy, Bowser falls easy. Just remember to heal when you're below 10 HP (Post-Kammy; 15-17 Pre-Kammy). Also, switch out partners if they're short on life, but use the partner to execute the switch, no Mario. And Sping Jump is awesome.

I'm close to where you are, about to kill Grodus. I haven't got to Bowser just yet but just from common knowledge I know moves like Supernova (star attack) and Firey Jinx (Vivian) will definetlely help, or items attacking all enemies at once.

Supernova can cause up to 15 damage, so if you're close enough to their defeat use it. Using tattle in battle (Goombella) will show HP, so you can see how close you are to killing them.

Hope this helps.

First, don't increase bp because you won't be able to equip any badges before Bowser shows up. What really helps is Sleep Stomp. It'll put one of them asleep for 5 turns, and you can laugh at them.

Don't attack whoever's sleeping, because they might wake up. You should use Goombella's multibonk also.

Use power lift with mario and then use stampede with yoshi and you can do like 30 damage to both of them if you have enough power plus p badges. You can kill them in 2-3 turns that way.

Start off the battle with power lift. First things first: destroy Kammy Koopa. Since she can replenish 8 HP, use really powerful attacks that do much more than 8 damage; power bounce and spring jump do wonders. Fiery jinx also comes in handy. Once Kammy is done for, use power bounce and spring jump on Bowser. Use power lift again whenever power lift wears off.

PS: If you think the battle against Bowser and Kammy is difficult, just wait until the final boss!