Ignition attacks are executed by pressing left trigger after you meter on the left fills and you enter an enhanced state. They are your "Ultimate" moves so to speak, and completely diminish the gauge upon execution. In order to pop this trophy with relative ease, it is best you build up your gauge by using standard X attacks and unleashing the move as soon as it's available. Breakable Shrines with the blue gems hovering above them occasionally offer a complete gauge refill which is red similar to the bar. This will pop as soon as you execute the 5th ignition attack.
If you'd like a combo which helps boost the gauge quickly, I suggest using Ichigo. Using his square x 4 combo in to his jump and then Circle repeatedly you can get a very nice combo going as well as a good boost to your meter. As soon as it fills, use the ignition and repeat.
The two nodes for improving Ignition Gauge rate are Ulqiorra and Hollow Ichigo. They are next to each other on the lower portion of the skill map. With both of them obtained, the gauge will fill up at an extremely fast rate making this that much easier.