Charge Attacks are when you sprint by clicking the Left Thumbstick, then pressing to perform a melee attack as you sprint. Charging orks like this twice will kill them, but a couple melee attacks or shot from a pistol or bolter before the charge should help make the charge attack a one hit kill.
An easier way is to charge through Gretchin (the little green orks) or through Chaos Guardsmen. Both have extremely low health and will die with a single charge.
To speed up the process, go to Chapter 13 and find the room where the Chaos Guardsmen are manning a Heavy Bolter. Kill them and proceed through the room. When you attempt to leave a Daemonhost will appear and begin summoning more Chaos Guardsmen. Here's your chance to grind this one out. Do NOT kill the Daemonhost, and just begin charging through enemies. There will be a never ending tide of them, so long as the Daemonhost doesn't die.