If you meet with Merlon, he'll inform you that the next Heart Pillar resides in Flopside Outskirts. So venture forth to Nolrem's house in Flopside, then go through the nearby gate in 3D. Stay in 3D to continue down the path to Flopside Outskirts where you'll find a large, yellow block. Destroy it with Cudge to reveal a warp pipe.
Now in the basement's first floor, flip to 3D at the sign to discover another path. Use Dottie's shrinking power to walk beneath the structure, then use Cudge to hammer away the large yellow block (visible in 3D), to reveal a red switch. Step on it to make the Heart Pillar appear. Now use the return pipe to quickly get back to the door.
Chapter 6-1: SammerGuy Showdown
Head right, and Sammer will come up and tell you about a tournament that he has going on called the "Duel of 100". The prize? A very needed pure heart. Beat all 100 of King Samson's warriors to win the heart. A simple strategy is to use either Bowser with Barry (if you have him) or Carrie. You can also use Peach with Cudge, or Mario with Barry or Cudge, but whatever works for you should do you well. When you beat the 20th, they will let you go by without a battle. Hit the Star Block after the 25th warrior.
Guards numbered 1-20:
- Jade Blooper
- Hill with Eyes
- Puffing Fist
- Squatting Birdo
- Leeping Cheep
- Master Wiggler
- Shoe of Kuribo
- Guy who Fry
- Screaming Mantis
- Koopa in Winter
- Footsteps of Coins
- Urchin Lung
- Crow Who Eats
- Swollen Tongue
- Slipping Grip
- Clammy Hand
- Forbidden Slap
- Hairy Arantula in the Grass
- Another Castle
- Rolling Thwomp
Chapter 6-2: The End of a World
World 6-2 may be the shortest one yet. Beyond a short boss fight and a few doors to run through, there's nothing else to do. However, the battle is one of the most difficult challenges yet.
After speaking with the king, open the chest on the pedestal to meet a most horrible fate…or at least that was the plan. The King, devastated by his devious plan failing will reveal her true self: Mimi.
Mimi
- HP: 20
- ATK: 2
- Reward: 2,000 points
- Difficulty: *
- Attacks:
- Rubee Shell
- Rubee Bullet
- Rubee Shower
- Flip
- Spin Tackle
Mimi's main attack pattern involves flying about the arena, hurling rubees. In addition, a shield of diamonds constantly orbits around her.
Use Peach to battle Mimi, as her umbrella shield is invaluable in guarding against her attacks. Press 
to use it whenever a diamond is flying toward you. Once her attack relents, use Thoreau to grab a diamond (either from the ground or her shield), then chuck it at Mimi. You'll have to take down a few of her diamonds first to reveal a hole large enough in the shield to throw a diamond through.
Mimi also flips between dimensions frequently. Although you may be tempted to switch as well, stay in 2D and just wait it out as she's much more difficult to attack in the third dimension.
After the battle, run through a few more doors until the Void consumes and destroys the entire world.
Flipside
After the Void consumes the world, you'll be spared a grisly fate and end up back in Flipside. Before proceeding, save at the Save Block near Merlon's, buy some stuff, then dart right back through the Chapter 6 door to re-enter world 6-1…only things are a bit different now.
Chapter 6-1
There's nothing here! Well, don't let that put you off - head to the right for a minute or two (while riding Carrie for a speed boost) until you meet up with Mr. L and his big friend after the minute or two. Don't even bother looking for hidden treasure - there's nothing to find.
Brobot L-Type
- HP: 64
- ATK: 2
- Max ATK: 5
- DEF: 4
- Fire DEF: 8
- Weakness: Explosions
- Reward: 6,000
- Difficulty: ***
- Attacks:
- Shockwave Stomp
- Crunch Vacuum
- Big Missile
- Plasma Beam
- Rocket Punch
- Boomerang Blade Launch
Brobot L-Type comes equipped with a variety of new features, including a bona fide set of hands and feet - the latter of which is his weak point.
Use Peach, since her umbrella shield is invulnerable to most of his attacks (with the sole exception of his inhale), and plant Boomer by Brobot's feet to deal some pain. Remember, you can ignite Boomer early by pressing 
after setting him down - make heavy use of this to time his explosions accordingly.
The brunt of Brobot's attacks consist of leaping around - just stay on the move to avoid these, and try to plant Boomer where you anticipate where he'll land. In addition, Brobot will occasionally employ a Kirby-style inhale attack, which will suck up anything nearby. Keep your distance, but if you're feeling particular daring, try to get him to inhale Boomer to do massive damage.
After smashing Brobot to bits, you'll collect a Pure Heart... or at least, what's left of one.
Chapter 7-1
Back in Flipside, go show the remnant of the Pure Heart to Merlon. However, your chat will be cut short with Dimentio's appearance. It seems he hasn't taken kindly to your progress, and will use the opportunity to transport the lot of you to the "Underwhere," an afterlife, if you will, for video game characters whose "games are over."
After arriving in the Underwhere, a friendly resident will give you the lowdown on what exactly this place is. Afterward, head right to a door on the far right side, and make sure to save at the block just after. Heal yourself in the fountain, save, then head to the door on the right. First, flip and go around the pipe, take the right path, over the spikes, over to the left path at the second pipe and reach the door.
When at the river, you can either swim across, or speak with the boat rower to ride across for four coins. It doesn't much matter either way, through swimming is slightly faster, and of course, cheaper. Either way, enter the door at the end.
Climb up the stairs to find Queen Jaydes. She'll offer you an opportunity to escape the Underwhere in exchange for finding her daughter, Luvbi. Make sure to accept her request, otherwise you'll face a Game Over screen after four denials. To aid in your quest, the Queen will gift you with the Door Key. Now backtrack through the door, back to the river.
Go back to the river you swam through and dive under to the bottom (flip to avoid the ghostly hands that try to damage you) and flip to enter a hole in the bottom of the sea. Use the key to open the door.
Swim through this water-filled passage in 2D. Ahead, three bricks block the way - since they can't be destroyed while in water, you need to find some other means. When at the pipe, swim up to find a large block; push it into the shaft on the right. Follow it down, and activate the switch on the wall to drain the passage of water. Now jump from the block, to the pipe, to the ledge on the right. Smash the three blocks there, then activate the switch again to refill the room with water. Now swim through the section where the blocks were to continue on.
Run to the dead end and flip to reveal a passageway at the right. Continue swimming across the large room to another passage on the opposite side. Look for another large block ahead. Once there, flip to 3D to get behind it and push it into the shaft. Now drain the water in the room by activating the switch in the bottom-right corner. Like before, use the block to reach the pipe, then the passageway above, and head back toward the large room you crossed earlier. Drop to the center of the large room and flip to 3D to find an adjacent path and follow it to the door.
The ledge to the upper-right is where you want to go next, but you'll have to use the fountains to get there. Cross the three geysers by leaping from the top of one to the other, to reach the high ledge. You'll find a cowering Luigi there. He'll be so happy to see his brother, he'll join you in your quest! Luigi's gift is that of jumping - his high jump will allow you to reach otherwise unreachable places. Before leaving, switch back to Mario and flip to 3D to find a Long-Last Shake behind a boulder. Now make your way back to the door on the left. Although it's too high to reach conventionally, Luigi's awesome jumping ability will do the job. Hold to prepare his Super Jump, then release it to leap to the door. Now backtrack to the large room.
Once there, use Luigi's super-jump to jump atop the pipe on the left and continue to retrace your steps back to the river. Swim to the surface and exit through the left door. Now make your way back across the room to the two pipes on the opposite side, flipping to 3D when necessary. Once at the two pipes (near the door), use Luigi's Super Jump to reach them, then the ledge above. Navigate around the pipes (by flipping to 3D) and enter the door at the end.
While you can attempt to leap over each pipe, one by one, spare yourself the effort and breeze by in 3D to find Luvbi at the end. After correctly answering her name, she'll return to the queen. Now head back to the queen to seek your reward. (Head out the left door, drop to the section below, enter the right door, then cross the river to meet the queen). Not only will the Queen give you the Revitalized Pure Heart, but she'll also return you to FlipSide.
Re-doing the Duel of 100
After you beat the game, you have the option of redoing the Duel of 100 and this time you can complete it. Chapter 6-1 has the first 25 Sammer Guys, Chapter 6-2 has numbers 26-50, Chapter 6-3 has 51-75, and Chapter 6-4 has 76-100. Upon completing the Duel of 100 Tournament, King Sammer awards you with 7 Catch Cards since he doesn't have the Pure Heart (you already got it earlier in the game). Each card has stats of the partners from Paper Mario: The Thousand Year Door.