A battle sequence is defined as from the appearance of an enemy until you beat the last enemy in this section (signified by a matrix style slowdown.)
You will have many opportunities to earn this achievement. If you have trouble, you can replay a section several times to learn enemy placement, or come back to an earlier stage after you've leveled up and gained some better gear.
This bit from Choskie in the comments is outstanding.
(Slightly edited)
This is insanely simple in co-op as player 1 can dodge roll (granting complete invincibility) and the other player(s) can kill all of the enemies. This method is more for impatient gamers, such as myself. I just HAD to get this on the first level we played!
Thanks, Choskie!