I got this earlier today by replaying Chapter 7 on normal and progressing to the third nightmare(?) sequence where you control a giant spirit form Korra.
When you get here, don't bother fighting the Mecha Tanks and instead focus on the Chi Blockers. After a little while you'll pick up a treasure item, which will be a cabbage. When it's been collected, let yourself get killed and start the sequence over.
Repeat this method over until you've counted 10 cabbages collected. Once you get your tenth, continue and complete the level as normal. On the chapter summary screen, you should see you 10 collected cabbages and the trophy is yours.