Rage mode is the state you enter when you get low on health And get that red glow. Rage mode ends when you use your rage art or ability. The damage you deal is cumulative. You don’t Need to do the 1000 damage in one battle.
This trophy Should come natural though gameplay.
If you want to get it out of the way sooner, you can Always enter practice mode And set rage mode to on. This way you’ll be in rage mode permanently