Here is a quick break down of Cecil starting with a quick character overview.
Cecil is categorized as a Vanguard type character. He isn't the fastest but he is capable of dealing high damage (some of the highest in this beta, upwards 500 points in some attacks) and forcing opponents in close spaces to eat Brave or HP attacks depending on his form. Cecil is a combination of a Dark Knight and Paladin as he has been in the past Dissidia games as well as in Final Fantasy 4. In this game you are able to choose which form he starts in when you are selecting him as a character. He is a difficult character to play due to the nature of his forms and when changing to another form you should be aware of what pros and cons you are taking up.
As a Paladin Cecil excels in air combat. He is quicker while running and dashing and has wallrushes and floor rushes on all his attacks based on how high he is when attacking his target. He only has one ground attack in this form and it isn't particularly powerful.
As a Dark Knight Cecil commands a large space on the ground and is extremely powerful here. He is weak in the air (only one brave attack) but that attack has a very fast startup and wall rushes.
His Unique Skill is Job Change. This skill takes 30 seconds to charge up and when initiated will change Cecil into his opposite Job (Paladin into Dark Knight and Dark Knight into Paladin). While changing jobs Cecil is immune to all forms of Damage and Crowd Control, this can be used to avoid Attacks, summons, or even status effects. He seems to retain invincibility for abot a second after changing so he is able to attack right out of the change which can catch opponents by surprise. For the next 10 seconds Cecil deals higher damage and some HP attacks gain extra abilities.
Luminous Sphere is Cecil's HP attack while in Paladin form. It is a straight shot with mid range but is very quick, traveling it's max distance very quickly. While powered up by Job Change it has slight tracking and slightly better verticality. There honestly isn't much to talk about right now, read below for more.
Tenebrous Blast is his HP attack as a Dark Knight. It is slow, has a much larger hitbox, but tracks the opponent until it either hits terrain and detonates, hits and opponent and detonates, or reaches the end of its travel distance and detonates. (Note that both HP versions will detonate at the end of their travel distance regardless). When initially cast it has the same travel speed at Luminous Sphere but only at a close rage (it is a good wall rush followup but only at a very close range). Tenebrous Blast lasts a very long amount of time, I've seen it last up to 15 seconds before detonating, which is a very long time. It has a very large explosion area. Probably the most important thing to know about it is that Tenebrous Blast will stay active even if Cecil changes to Paladin form and even if he uses Luminous Sphere. Two HP attacks coming at you from the same target is a HUGE thing, that's a ton of pressure to make them panic.
Both HP attacks can deal Brave Damage before exploding and dealing HP damage. It deals 240 Brv damage and 456 while Job Change effects are active.
Holy Flame and Dark Flame are his other HP attack options in this beta.
Holy Flame creates an explosive area that deals HP Damage after a delay. It has a medium Range and covers a wide area. When successfully hitting an enemy Cecil will gain HP (about 20% of total health). The only iffy thing with this HP attack is that the attack doesn't seem to become active until about a half second into it's explosion animation and it is very easily escaped.
Dark Flame is very much like it's old iteration. A pillar will shoot out from Cecil and track to an opponent at a fairly close range. It costs 400HP to use in this beta, no matter if it hits or not. The HP reduction occurs as soon as the pillars become active, about a half second after starting casting it.
Both HP attacks are capable of hitting multiple opponents and gain no extra effects while under Job Change.
A note about both HP sets. These are not guaranteed wall rush followups. The startups are too slow but there are ways to greatly increase the likelihood of connecting with each one from a wall rush (almost strictly in Dark Knight form though).
Dark Knight's standard X string has a huge hitbox, even hitting enemies just overhead of him. It wall rushes and is extremely quick. Pretty minimal movement during the whole attack.
His forward X is a new form of his Dark Lance. Instead of physically throwing a spear he now just shoots it out of his sword. Deals high damage in a single hit and has a medium range (also has an absolutely enormous hitbox). Slight vertical tracking.
His back X is more of a utility attack rather than an outright attack. You create a large dark area that will detonate after a time. The amount of damage it deals and how long it lasts will increase based on how long the attack is charged. This attack does have a vacuum effect, making it perfect to trap someone and hit them with an HP attack. The weakness of this attack is that you have to charge it (you can just tap it) to make it effective and opponents can just dash out of it almost instantly. The absorption effect is too strong to just run out of it.
This is your big pressure tool outside of Tenebrous Blast and sets you up for successful Tenebrous Blast hits after close range wallrushes. Wall rush, Back X (any charge really) Tenebrous Blast in that order and your opponent is guaranteed to panic. You've just created an enormous are of "fuck you" they have to deal with and this area is a monster to deal with. The absorbtion of Back X prevents them from falling or dodging away, they are forced to dash and they have to dash in a direction away from you as Tenebrous Blast comes from you. In a head on wall rush you are damn near guaranteed a hit.
His Dash X is his old Gravity Ball. When he hits it will send the opponent to the ground and floor rush them. Little tracking.
His air X is a very fast three hit string. Covers small area but wall rushes.
Paladin excels in the air.
Grounded X is a quickly two hitting string that floor rushes the opponent in open space and can wall rush when close to a wall. In open space it knocks them VERY far away from you, you won't hit with shit at this kind of range.
Air X is his old Sacred Wings but with a better visual on the second portion of the attack. Floor rushes in front of Cecil but can wall rush when very close to a wall.
Air Up X will make Cecil charge upwards and the second part of the string will send the opponent downwards. Wall rushes and can floor rush.
Air Back X will send Cecil attacking downwards and knock the opponent upwards. Wallrushes.
His dash X is his old Searchlight. This extends to its max distance and will fire three light attacks. These will not stun but deal a good amount of damage. Can confirm it has no link bug in this game.
In the full game Cecil will have his old Paladin Force and Soul Eater HP attacks which will allow him to change Job by using his HP attacks (Paladin Force forces you to chnag to Paladin and Soul Eater forces you to go to Dark Knight). I don't know offhand whether or not you have access to both or just one, as of right now I would say Paladin force is the superior of the two due to having a better range. These can be guaranteed wallrush finishers.
Right now Dark Knigth has the better damage options but Paladin meets the "air game"that the game is heavily reliant on. I recommend starting in Dark Knight form simply to immediately apply pressure with Tenebrous Blast and Job change as needed. Dark Knight feels more powerful right now because action is a lot closer to the ground than it was in past games (where almost all you would do is attack from the air).