*Refer to Lavindathars guide on how to attain rage mode and how to use the ability*
Once you attain rage mode and have maxed all rage abilities head over to the final mission. On easy, during the first waystone fight, kill enough enemies to fully power up the rage bar. Once that is done switch to brave difficulty (to die faster) and get killed. Now when you respawn you'll have a full rage bar. Switch the difficulty to easy, go into the fight, kill a few enemies (ignoring the giant rock-throwing fire golems) and wait for the exploding enemies to spawn. Now activate rage and kill all the enemies except for the rock throwers.
NOTE: The reason we keep them alive is to not only die but to also be able to spawn with a full rage bar. Quitting out of the game and joining back in spawns you back at the start of the fight but also spawns you with an empty rage bar. Also, I found that jumping off the map is much slower than getting killed by the rock throwers.
The rage ends pretty quickly, so when it ends start killing the rest of the enemies filling up the rage bar again. Once its almost full switch to brave difficulty and get killed by the rock throwers, this should fill out the rest of the bar completely. You'll spawn with a full rage bar, switch the difficulty to easy, follow the steps above, rise and repeat.
TIPS: Use your bow with the wind charm. I found the exploding minions from the earth charm to be too much and they kept destroying the rock throwers. Each playthrough should be able to rack up 10-15 kills, and it is pretty quick.
Hope I helped!