I just want to do something so damn simple, so very simple.

CACTUS:
  - trigger: LEFT_CLICK
    damage.attacker: 2

I've tried no trigger, I've tried action: BREAK, I've tried trigger: BREAK.

I can't do anything to make this Cactus deal 2 damage on hit or break. It's driving me to my wits end. Please tell me, what I could possibly be doing wrong.

I fixed your formatting for you. No idea how to do what you want though, since I haven't worked on this for a long time now.