The certain number is exactly 100, this information will show when you've unlocked the achievement.
Unlimited mode is activated by pressing "LT" when you've filled up the gauge enough, not to be mixed up with the "Burst" bar though, this one is below it ! When in unlimited mode your Heavy attacks won't need a cool down, you do more damage and you take less.
A fast way could be spamming Heavy Attacks ( Y ) when fighting large groups of enemies. Another fast method is to get into unlimited mode when doing the on rail shooting parts of the game and using the lock-on "Y" attack.
You will probably end up unlocking this without intentionally going for it on your second playthrough.