What you see is an approximate 8" gap between two cinder block walls with a Brazilian Pepper growing between them.
It's an aggressive grower that is on my top list of despised plants in southern California. Good luck killing that tree off. Your puny attempts to cut it will do nothing to its plan for yard domination. To those who think that herbicides are never called for, I tell you that if I had this situation I'd be trying to import banned herbicides.
Failing that, I'd set up some sort of herbicide IV drip.
Don't put yourself in the situation where you have to deal with this situation.