I couldn't finish 'em. Looks like this's gonna get complicated.
Diabolos uses Dark-based attacks, most of which target the entire party. Assuming the party has the best gear from Lufenia, his normal attacks can do up to as much as 1,100 damage. Diabolos can hit twice in succession for a whopping total of 2,200 damage and change.
Diabolos uses an ability called Dark, which can do up to 1,200 damage without Shell and uses Evil Messenger, an attack similar to Dark but stronger, as it can hit the entire party for over 1,200 damage each. Diabolos also has a similar move called Dark Breath.
When Diabolos is about to lose, there is a chance it will use the devastating Soul Despair move, which puts everyone in a near fatal state. This move is equivalent to Tornado, but affects all targets.
Dark-type moves will heal Diabolos, including Dark Knights' attacks.
If the party has a Dark Knight, they should utilize their Draw Attacks ability to have Diabolos target them.
Diabolos is the Greek word for "devil". It has entered many languages to mean devil, such as Diabolus (Latin), Diavolo (Italian), Diablo (Spanish), Diable (French), and Diabo (Portuguese). Diabolos actually means "accuser" or "slanderer" and could also be connected to the Greek word diabolous, which means "divider" (which fits given his affinity with gravity element), but eventually the general word Diabolos became the specific name of the entity. In the original Greek rendering, it was used to refer to the Christian Devil (The New Testament was written in Greek). Devil is the English translation of Diabolos and in Christian belief, this being is the embodiment of evil.
The concept of the Devil is believed to originate in Zoroastrianism with Angra Mainyu (also known as Ahriman) as well as from the Judaic Satan in the Book of Job in the Old Testament. However, their Satan was merely the "devil's advocate", an angel who acted as a skeptic and whom God allowed to afflict Job with suffering. Satan is Hebrew for "prosecutor/accuser" or "adversary".
The concept of the summon Diabolos may be connected to the Jinn in Islamic mythology, similar to Ifrit. The djinn were the origins of genie myths and were spirits or ghosts made of fire or smoke. They were said to grant wishes.