Just like all of the other combat maps, it's based on how well you perform in combat, the only difference is that there's a timer, and constantly spawning enemies, instead of there being a set amount of enemies and no timer.
Your level, which you can use to "bank" increases based on your score since the last time you were hit by an enemy, or since you last banked.
Since it's based on how well you've been doing without being hit, it doesn't really matter whether you choose to take out as many enemies as you can the quickest, or if you choose to make the most out of each enemy. Just do whatever lets you build your points up the fastest.
I personally find it fairly easy to vary the combat while still taking enemies out quickly.
Just build up your combo and use both quickfire gadgets and special moves to get multipliers that will increase your score dramatically.
Levels are unlocked incrementally starting with level 1 at 1000 points, and increasing from there (Level 2 is either 2000, or 5000, I can't exactly remember).