Just posting a few things I've learned while playing (and using cwcheat ) with this demo:
I've been using EX Revenge a lot, to get more familiar with the system. To give you an idea how it works.
Once EX Revenge is activated you have something like 10 seconds to do anything. The interval will end the moment an HP Attack is triggered...BUT...you can optimize it by doing up to two Brave Attack combos followed by an HP Attack to really stack on the damage.
For example, Cloud can do Slash Blow > (wall hit) Slash Blow > (wall hit) Slash Blow (interval ends) Omnislash Ver. 5 all in a single EX Revenge interval as a combo, this is with the aid of walls, plus the interval ended mid-combo, but it was still finishable since the enemy was in hit-stun.
There's more: EX Revenge changes the properties of most Brave attacks, so it completely ignores moves with huge recovery (except HP attacks) and chase properties, allowing you to link chase moves together with other moves for some really stylish combos.
Here's a really neat one I invented with Kain (triggered with Mid-Air EX Revenge):
Celestial Shooter (first two hits of the combo where he moves upwards with his spear Up+Circle move)...Forward+Dodge-Cancel...Sky Grinder (Down+Square).
At the height of the top of a stage, this can grind the opponent almost all the way down, giving you tons of Critical BP in the process.
There's others. Lightning's Blaze Rush (COM) can combo into itself up to three times almost anywhere on screen during EX Revenge, and even better, Army of One (RAV) can combo into itself twice if you time it right, since EX Revenge ignores chase properties.
Also, I'm trying to confirm this, but it seems that using a Level 2 Assist (L1+Square) when being attacked may not necessarily give you an Assist Break, only reduce your Assist to 0. Also it seems that Assist Break can be triggered if the opponent manages to block your Assist's Brave Attack in time, not necessarily attack.
Worth noting that some assists really do help with complicated HP attacks, such as Cecil's Air Brave combo that leads to knockdown, giving Kain a free Jump HP Attack. Tifa's Chase Launcher Assist can give Cloud a free Omnislash Ver. 5 if you press X to follow up her chase as well, Assists can serve as a good distraction for an opponent if you're trying to get a clean hit in too, or if they run away too much, you can simply use Assist HP Attack to get the job done for you, even though it's risky.
Hope someone can put this info to use.