In the English rules, on the sheet that explains how each card works, it mentions that the Chief Outlaw cannot be defeated by using activity cards. It must be defeated by taking on the event card itself. I believe this is the only event card that you cannot put a cube on from activity cards.
I really wish they had placed some sort of symbol on the card itself to let you know you can't use activity cards for it, or at least some way denoting it is special.
Great, thanks a lot for your comment ... you're absolutely right !!