This demon... nearly tops the annoyance list (Kraken is the worst...)
Diablo, a demon partly unsealed from some kind of floating gem...
He's Lv.66 and the last boss of the game (Hopefully*)
He is not the hardest boss I've seen in RPG's but he is definitely a nuisance. If your are Scouts, stick away from the element attacks (Unless you're confident your dodge and health is high enough) Warriors same thing but you are the pot chugger any but be watchful of stacking damage, and Tanker, well not much I can say because you're a class based off of defence.
At the start, he's the easiest boss, he just sits there staring at you. Moving towards you after some time only to clap you in his hands. Though once you drain 1/3rd his health he will disappear and two smaller Diablo's appear.
Once again the Mini-Diablo's until the main body re-appears again
Now, he's just going to give you a bad day when fighting for the first time. He will spawn 4 Golems of various types, but they are all the same difficulty. Which is actually harder than outside this boss battle. Yes you can defeat them, but it's a massive waste of time due to Diablo spawning in 4 more after you kill them all. Even if you only kill 3, he'll just spawn 3 more. So don't bother.
Next part, he teleports from each corner of the map to another corner. Will not move towards you, you have to continuously chase him down. If you run for point to point quick enough and smack away, the golems will not catch you. However Diablo will not punch you anymore, but stacks upon stacks of elements will be thrown.
Best piece to top it off, Diablo introduces a 4th element. Gravity. A black ball with a purple ring, will suck you inside and deal damage, as long as you are not inside there will be no damage. This power is rarely used however, only if you're not chasing him down and fighting the 4 golems will he use it on you. Even then, it's quite rare to happen.
Lastly, attack patterns. Same deal with the Mini-Bosses, distance from him determines the attack type. Though, because it's the last boss, there needs to be an overkill of element usage. Not only that but from the amount of teleports, he's recovery time is hard to manage. Save the SP pots for dashing.
The way all versions of Diablo will attack:
(Not 100% correct, more of a majority use)
Both Mini-Diablos are on one side of the map, you on the other. Expect a ton of Fire Pillars and maybe a bit of Lighting.
One Mini-Diablo is on one side, you and the other are on the other. Expect Fire Pillars from the far one, and Ice Swirls and then Lightning Chain upon teleport from the closest.
If all of you are within gun distance, expect them to close in and use Ice/Lightning Attacks then teleport away leaving Lighting stacks.
Stage 3 version only:
Fire Pillar Rows (Each row has 5-8 Pillars, up to three rows)
Ice/Wind Swirls (Up to three will be blasted before he teleports)
Lighting Chains (Just a couple of stacks, only upon teleports or at gun distance)
Gravity Well (After a set amount of time not attacking Diablo, he will then use and overuse these)
Until death you'll need to aggressively engage this battle. Once defeated, loot the invisible corpse (Sealed back into the floating jewel) and head for the doorway at the top right. You will be automatically brought into threater mode for about 5 minutes.
*Why do I say hopefully Diablo is the last boss? Hell Mode is broken, so we do not know the other half of Wild Frontier. For the Next/"last mission", please click on "Very Bad News People" at the bottom on the guide tool at the top left page navigator on this site.