Phantom Breaker: Omnia

Hey this guide Is not correct,here is extra information from the PlayStationstation guide Homepage
In addition to Counter Bursts, another type of burst exists in the form of a Critical Burst, which is a very powerful attack that is unblockable and deals a significant amount of damage depending on the amount of meter you have accumulated throughout the match (so for example, if your meter is full, then your Critical Burst will deal a lot of damage to your opponent). Keep in mind that this type of burst can only be triggered when your Tension Gauge (which is right next to the current round's timer) is full.

In order to increase the Tension Gauge, you and your opponent will need to do any one of the following options since this gauge is shared between both players:
Holding a Counter Burst (hold cross.png) for a long period of time.
Your's and your opponent's attacks clash with one another (which provides the best increase).
Walking or dashing towards the opponent (the former increases it by small amounts, while the latter provides a better increase).
Once the Tension Gauge is full, a timer will start with five seconds to boot once this gauge fills up for the first time (the timer will increase if this is triggered multiple times during the course of the match), and once you hold cross.png for a prolonged amount of time while this state is active, the background will turn into a darkened red, which will trigger the Critical Burst. An example of this can be seen in the screenshot below

I couldn't get this to work in training mode. Went to vs mode and didn't use any meter, just normals, til 2p was close to dead, then I held A (which had armor for a couple enemy attacks) and the attack hit and killed. I think the "KO" needs to appear for this to count, which doesn't happen in training.