The eleventh Tablet can be found during Darkness over the Hanshui River, which is the ninth main mission. After crossing the boats and water, you'll be on dry land and heading towards a guarded door. To get there, you have to pass by a Shuigui mermaid. On the right of her is a little boat, with a Warlock and Demonized Officer guarding it. Once you're on the boat, drop down and head to the back to find a small box where the Tablet is.
The twelfth Tablet can also be found during Darkness over the Hanshui River. From the last Battle Flag, go to the right. There's a house on the left, immediately after the barricade. Go inside here to find the Tablet.
The tenth Golden Cicada Shell can be found at the start of Darkness over the Hanshui River, which is the ninth main mission. From the first Battle Flag, go towards the water and then go right. You should see a Shuigui in the distance. Head toward the mermaid, slay it and then go to the furthest point you can go beyond her. There's a boat on the left, if you jump on this and look down you'll see the Golden Cicada Shell.
The sixth Dragon Vein Essence can be found during Darkness over the Hanshui River, which is the ninth main mission. Just before you reach the final Battle Flag, which is located on the right in the image above, you can pick up the Dragon Vein Essence. You can do this just before jumping over on the boat to reach the Battle Flag. The small cabin marked on the picture shows the balcony you need to use to jump inside. Once in there, you can find a chest with the Dragon Vein Essence in.
The eleventh Shitieshou Demons can be found during Darkness over the Hanshui River, which is the ninth main mission. When you've crossed the water and are on dry land, you'll come to a guarded gate. Inside, you'll have to take a demon Battle Flag. Opposite this Battle Flag is a destroyed shack, but the little panda demon can be found inside.