This is cumulative and will probably come naturally over the course of playing through the campaign, Horde and Score Attack. Low health means you need to be in the red portion of your health meter for 20 combined minutes. The game is very liberal with letting you survive, even at low health, so if you're careful as you eliminate enemies, you can remain at low health for a very long time. Just remember that health potions restore your health, so it is recommended that you don’t use one unless you absolutely need to.
To see how long you've been at low health, select 'Extras' > 'Career Stats' from the main menu.