Super Mario Galaxy 2/Bowser's Lava Lair

Bowser's Big Lava Power Party
On this simple platform, just defeat the Magikoopa that pops up and a pull star will appear in the center of the fire circle. Grab onto the pull star with and it will take you far enough for the gravity of the next planet to take effect.
 * First planet

This is a cylindrical stone and lava planet. The first lava section has moving wooden platforms you can jump on, but beware of the lava that shoots up periodically on the sides of them. Ground pound on the springboard on the other side to go to the next planet.
 * Second planet

This is just like the last planet, except more dangerous. Avoid the fireballs circling the planet by running alongside in the same direction and cutting through one of the gaps or just jump over them. In the next lava pit, the boxes move back and forth across the lava while slowly circling the planet. On one side of the planet is a Life Mushroom over the lava, while on the other side is a 1-Up Mushroom. At least grab the Life Mushroom while crossing by boarding a box when gets to your side and letting it take you across. At the top of the planet, break the crystal and use the Launch Star inside to proceed.
 * Third planet

Grab the checkpoint, then use the flipping platforms to cross over to the Hammer Bro., optionally picking up the 1-Up Mushroom on the out-of-the-way platform. Just jump on the platforms like normal, but if you see it flash an angry face, quickly spin in the air to delay your landing while it flips. Also watch out for the fireballs that shoot out of the lava while you're crossing. When you get to him, run sideways and toward the Bro. to avoid his hammers so you're close enough to jump on him after his second one. Once he's on his back, just run into him to defeat him and reveal the Launch Star. The Launch Star take you up to a platform with a large three-pronged fire wheel with a ? Coin above it. The ? Coin reveals a circle of coins going around the fire wheel, so jump down between two of the spokes and follow the wheel around to collect them and get a 1-Up. When you're done, ground pound the red button on the other side of the platform to down below.
 * Fourth planet

There are two Dry Bones down here with three more spinning platforms crossing the lava to the other side. Once you're across, you'll face two Whomps along the green path. The easiest way to get by them is to run at them and crouch between their legs. When they see you they get mad and stop, preparing to slam down on you. If you don't want to go through their legs, just back off when they stop and you can go over them after they slam down. Optionally, you can ground pound their backs for star bits. On the other side of the Whomps, jump up onto the meteorite and ground pound it into the lock on the large door in front of you. When it breaks, the doors will open and you can run forward to be sucked through to the next planet.

Grab the checkpoint flag and proceed into the area with the Mattermouths. Try and stay on solid ground, although you can jump on their heads if you absolutely have to. There's a 1-Up mushroom at the other end, and then you're past them and upside down. Continue along the green path around to a purple platform. When you get on it, it starts moving so keep up with it as it moves over the lava. You'll need to dodge some fireballs and a fire wheel while you're on it, although you can pick up another Life Mushroom here if you need it. At the end of the ride, jump on the Magikoopa and head back onto solid ground. The path will take you to another purple platform that goes between statues that shoot out an electrified line across your path. Simply jump over the lines and get off on the other side to face two Hammer Bros. Use the same strategy here where you close in on them from the side so you're in position to jump on them after they throw the second hammer. Continue on and grab another checkpoint and another Life Mushroom at the top of the flagpole on the left, should you need it. This time there are three locks on the door and you'll have to ground pound the meteorite off-center to get it to go into them. When all three locks are destroyed, you'll be sucked through the door again.
 * Fifth planet

This time you end up in Bowser's throne room and he's huge. After talking with him for a bit, you'll get knocked up to a new planet. Here Bowser floats in front of the planet and attacks you from above. First he rains down meteorites on you and you see where they'll land by the circles of light they make on the ground. Avoid those and he'll begin circling the planet shining a very large circle of light down. When he catches you in it, he'll stop and do a huge punch into the ground. This punch will raise the meteorites out of the ground, so make sure you away from them so they don't get destroyed. Ideally, you want him to hit as close the meteorites as possible without destroying them for the best shot at him. When the meteorites come out of the ground, jump onto one of them and ground pound it so it flies into Bowser. You'll have to geusstimate how off-center you need to be and in which direction so it actually hits him, which is why you want him to be as close as possible.
 * Bowser

He does the same series of attacks a second time with the meteorites followed by a huge punch. Hit him again and you'll see a short cutscene where he gets really mad. Now after the meteorites fall, he'll chase you around the planet breathing fire at you. After the fire he'll punch again, so lure him close to the meteorites and punish him with one of them again. Do this one more time and you'll have defeated him and gained another Grand Star, which will open up the next world.

Lava Lair Speed Run (Prankster Comet)
For this star you'll have 30 seconds on the clock and you'll need to get through the galaxy, although you don't have to fight Bowser at the end. Along the way you'll want to keep your eyes open for green bonus timers to add 10 seconds each onto your countdown. Start out by grabbing the first bonus timer, then continuing on and taking out the Magikoopa and using the pull star to get to the next planet. Go to the other side like normal and grab the bonus timer before taking the springboard to the next planet. If you think you'll need it and won't waste too much time, grab the Life Mushroom on the way across this planet, then get the bonus timer at the top where the Launch Star is.

Grab the easy bonus timer before the flipping platforms and quickly make your way across to the Hammer Bro. After you defeat him, the Launch Star will take you through another bonus timer and you'll find another one waiting for you above the center of the fire wheel. Ground pound down to the lower level and take the flipping platforms across and grab the bonus timer there. No time to waste here, just run up to the Whomps and slide between their legs with. If you can quickly long jump out to the timer on the left and back go for it, otherwise just skip it so you don't end up wasting more time than it's worth. Likewise, it's probably not worth stopping to get the 1-Up Mushroom above the second Whomp. Once you're past the Whomps, grab the bonus timer and break the lock on the door like normal.

There are two bonus timers in the area with the Mattermouths, and you'll probably need both of them so don't pass one up. After them, you'll pick up one more timer on the first purple platform and another on the second purple platform. You really don't want to get hit here, so take your time negotiating the fireballs and fire wheel, and be careful getting past the Magikoopa. After the second purple platform you don't have to go any farther—your Power Star will be waiting for you right when you get off.

Green Stars
The first Green Star is behind a rock in the lava next to the Dry Bones between the two Whomps. You can just barely see the rays from the star peaking out from behind the rock. You can long jump right out to it, but if you hit the lava just try to guide yourself so you hit it coming back down. The second star is underneath the Mattermouths after you go through the first gate, and it should be pretty easy to spot since the floor is being constantly eaten away. When you see a path has been eaten over the star, just jump right on it.