You can get this at the start of Mission 2: Blood and Rust
where you will fight 6 Titans. First let Lt. Shaver and Lt. Freeborn get destroyed. Shoot the Titans to build up your Burst Core ability to 100%. When it is ready, damage 3 of the Titans to about 25% health. If there are less than 3 Titans you will need to advance for more Titans to drop. Then line them up and press
to fire your Burst Core. Obviously this is easier if you set the difficulty to Easy.
There is confusion over what to call the episodes/chapters/sections/missions/checkpoints, so I am including them here for reference.