This is so easy!
1: Slow down time using healing touch
2: Take out all thorns before triangles
3: Once all thorns are gone, take out as many triangles as you can BEFORE the thorns come back. If ONE THORN comes back and you pull out a triangle (doesn't matter if it's on the opposite side), it multiplies.
You should get it all before the thorns come back, but if you don't, you can't slow down time again and just have to be fast. You should have taken out enough triangles already to finish it off. If it helps, try taking out a few thorns before using the healing touch.
Trust me, I've beaten this game and the DS version too.