Some information gathering from reading the lores and the latest manga chapter.

• The downfall of the 12 guardians was planned since they, along with Guardian of Meirith in Ishgria, were the closest group of humans to the gods. Even though they're all powerful, without Sodis and Ramna (and by extension, Lunaris) the "hearts" that connect to everyone together, it meant the guardian's destruction. Sodis, like Claire, is too powerful even for the gods and so he's the first one that must be eliminated.

• Melord, under Lucius' order, manipulating the events of their destruction to allow Ark's group to be able to attack Karna Masta directly, like he later did to Guardian of Meirith. Since Karna Masta already planned to destroy humanity anyway, this gave more weight to his words.

• Farlon and Luly are, the two most "instable unstable of all the guardians". Farlon for being disowned by his family, and Luly for being abandoned by her parents and being young to the world in general. Separating them from their lover/mother-figure (namely Ramna and Lunaris) will sure to instill some instability to them and it sure did.

• Ophelia and Zephyr are also another threat to the plan, since their personality mean that they're likely the one to find out the truth first and stopped the conflict. That's why Melord killed Ophelia (who found the truth first) and probably targeted Zephyr to destroy any evidences that they'd gathered.

• The rest of the guardians are in-between:either loyal to Sodis or Alyut. They're not as much threat to the plan (except probably Rina who was so loyal to the gods) so getting rid of them is bonus . Any survivors will be killed by the rest of the gods when the truth come to light.

• Summary: Sodis/Ramna dies, Farlon/Luly goes insane, guardian's annihilation, any survivors killed by the gods, Kill Ophelia/Zephyr and destroys all evidence so they couldn't reconcile.

• Miscalculation in the plan: Signas and Zephyr's survival. The former warned the six heroes, and the latter led to the founding of Akras Hall.