Attack on Titan/Tales of Symphonia Campaign Hybrid

I'm currently GMing a kind of hybrid campaign between Attack on Titan and Tales of Symphonia. This campaign is fairly high leveled (PC's started out at lvl 8), but I gave them super high ability scores. Here's the background info I've given my players up to this point:

'The three walls of Sylverant are the only thing you've known. Well, that and the Titans people talk about on the outside. Expedition after expedition, members of the Liberation Regiment come back fewer and fewer. More than 60% of the LR get devoured in their first hunting. So little is known about these Titans, other than their attraction towards eating humans. It doesn't seem like they have any ability to reproduce, and they don't seem all that intelligent. But, from what LR members have said, these creatures are absolutely terrifying. They move slowly, almost like a newborn learning to walk. But once they've spotted a human, they break out into a sprint to chase you down, wanting to devour you. The LR members still alive at this point are considered to have an extremely high survival rating - but no one is exempt to error. 'One is all it takes,' as is commonly said among LR members, many of whom who have lost close friends to these Titans.

The Liberation Regiment has instigated a draft for the defense of Sylverant. Every able-bodied human, at the age of 15, is forcefully required to apply. Many of these applicants do not survive the intense, three-year long conditioning process. Many merely turn into the equivalent of a housemaid for those who were actually formidable enough to pass through conditioning and into skills training. Based on your excels in conditioning, you will be discharged from the Conditioners and handed over to the Skill Experts. Skill training is broken into three categories: Melee, Ranged, or Magic Combat - of which all will learn the same baseline survival skills. The mental strain and turmoil applied to Trainee's is almost unorthodox, but the LR's requirements are set high for a reason. Only the best of Sylverant's gene pool will be admitted in order to maximize the percentage of success against the Titan's.

You brave souls are nearing the end of your training process and have been placed within the top 10th percentile of your respective skill training categories. You will soon be issued official Liberation Regiment equipment and gear, much better than what you've trained with. You, now almost officially under active duty, are planned to be included in the next expedition under the command of General Murdock. Strange Titan activity was reported outside the south-east end of the outermost wall from the last expedition. These Titans were not behaving as was thought to be traditional Titan behavior, merely standing outside the wall in place and panting. Your superiors have deemed it more important to capture these Titans for testing purposes instead of completely eradicating the group. Studying this bizarre Titan behavior may give a better insight than what we currently possess.'

They've just completed the first Titan (have been using Giants) encounter, so I sent out this little piece as a precursor before the next chapter begins:

'Comrades,

Congratulations on surviving the Titan onslaught on the south-east outermost wall. We needed to officially test your combat abilities in a real situation. You all proved very effective, although you're combat methods will require more honing and precision if you wish to succeed.

I have commissioned you all to complete a greater task and have sent someone along to help you accomplish it. This information is kept confidential, and we expect it to stay that way. Most of the cadets are needed for the direct defense of the city; sending other troops would merely be a waste of valuable resources in our eyes. Most would perish under the circumstances you will soon encounter. But you are all different.

You may remember a small scroll of parchment, signed, 'The Man'. There is nothing to worry about. This came as a direct order from me. Cardas isn't very skillful or tact with language or convincing, but he is dedicated to the cause. I have known him for many years, and I trust him with my life.

You have also received 8 teleportation scrolls along with a 6,000 gold stipend (each) to help you prepare for the upcoming mission. The LR's resources are limited, but we give you what we can. You're goal is to collect the 4 Elemental Devil Arms, which are currently holding this devastating curse in place. We have tried many times to gather these weapons, but all who tried have tragically failed. It is only by bringing these weapons together that Cardas can lift the curse. Never have we seen a newly commissioned group of cadets entirely survive their first Titan encounter. This is why you are all very important to the cause. There is a bond between you all that I've never seen before and it must be utilized.

You will be the group to accomplish the task - I can feel it. When completed, not only will the curse be lifted, but you will be rewarded very handsomely for your efforts. Peace will once again flourish abundantly in our city.

You can expect to find yourselves in difficult elemental terrain over the course of these missions, so prepare yourselves accordingly. Based on previous attempts, we've gathered enough information to determine the difficulty of each terrain (in order from easiest to most difficult):

Earth

Ice

Water

Fire

Today is the day that those who write history will start the chapter of the true Liberation of Sylverant.

Await further instruction.

Cradshaw, Rank IV Chief General, Liberation Regiment'

They had kind of an awkward run in with Cardas (I hadn't thought far enough ahead to roleplay his character very effectively), and I was kind of stumped as to where I should take it now. I was thinking of having them gather all of the Devil Arms together only to have General Cradshaw be Cardas (JuDAS IsCARiot) in disguise. He would then attack them, having powers from each of the Devil Arms. Classic betrayal story.

This campaign was designed to be difficult with definite TPK potential.

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The first one (what I've written so far), is called The Mountainlands:

I've devised a map with 15 points of interest (label with x's and the corresponding number). Certain x's need to be completed to be able to enter the next part of the dungeon.

The party warps in to the dungeon. It's devised of 2,000 ft high canyon walls with roadways in between, all under an open sky.

x1: Immediately upon warping in, the party comes into contact with a horde of 6 small earth elementals and must be defeated (real easy).

x2: A small crystal on a pedestal will be sitting in the middle of the area (a door is blocking the next area). Whichever party member (and only that party member) touches it will be teleported to the isolated area of x2(1) on the map and must solitarily defeat a xorn. The other party members are then left to defeat 4 large earth elementals that break free from the surrounding cliff rock. All enemies must be defeated to return the party member at location x2(1) back to x2. The door blocking the area opens.

x3: Chest with a wand of cure moderate wounds (8 charges), earthquake spell scroll with 'best to save this for later' inscribed on it

x4: Falling block trap

x5: Magic missile trap

x6: A mimic is disguising itself as the Devil Arm. Grabbing the sword turns it to life, along with warping in the monsters found at x7 (2 shaitans). After defeated, the next area becomes available.

x8: Chest with a wand of cure moderate wounds (8 charges)

x9: A large stone monolith (30 ft) with strange writing carved into the stone is seen in the middle of the area. Touching it will cause stone to crumble off of the top (falling rock trap) and also reveals 2 clay golems to fight. A wall shoots up from the earth and entraps the party members in the area. The wall crumbles once the golems are defeated.

x10: A large lever is jutting out of the side of the wall. Once pulled (simply), the lever falls to the ground and reveals an hourglass (for this segment, i'll be using a real, minute hourglass). Four buttons also emerge from the wall. After any four button combo, the party gets a magic missile TO THE FACE (4d4 +2). The timer then resets. Victory can only be won by allowing the timer to run out. The door then opens, revealing a chest. Upon opening the chest, a bone devil jumps out (i modified the bone devil to be able to paralyze with fear aura). Once defeated, the chest reveals a hearty surprise (which they'll need to the following encounters): Wand of Bull's Strength (1 charge), Wand of Bear's Endurance (1 charge), Wand of Mage Armor (1 charge), Wand of Blur (1 charge), and Wand of Summon Dire Bear (self created, lasts one encounter).

The party then moves into the next area which will take some more activations to have the next encounter happen

x11 & x12: Strength checks of DC 23 to move stone buttons into the earth.

x13: Essentially the area before they enter x11 & x12, but after the stone buttons are activated, and the party walks in to the respective area, four greater earth elementals break free from the surrounding cliff and fall to the ground, surround the party (crumbling stone will hit any party member within 10 ft of a cliff for +15 (7d6) damage).

x14: Once the elementals are defeated from x13, a chest appears before a gigantic door. Another wand of cure moderate wounds (8 charges) is found inside. Also, inscribed upon a button-like device, an ellipsis in quotations (in stone language). Activating the device will cause the doors to swing wide open, allowing them to the last encounter of the dungeon.

x15: Upon entering the area, only a small, singular bat can be seen fluttering around. Once killed, 5 stone giants can be seen emerging from the back wall, one wielding the elemental devil arm (the elder, who will have more hp and deal more damage). The elder begins to fly, lets out a scream, and turns invisible. The ground turns into mud, which halfs the movement speed of the PC's on ground.

Once defeated, the Elemental Devil Arm (which also will require a will save to resist it's evil effects) may be collected. Teleport back to Sylverant.