An oak will do a good job on the bat. I would suggest a metal bat. What you dont want is the bat spintering incorrectly and severing your jugular or whatever. Also have someone there with you. Actually any tree bigger than you is ok. You will not be able to hurt the tree but the satisfaction of thinking it is your perp or perps is unbelievable. I found I swung myself raw and screamed myself hoarse and finally bawled my eyes out. It was so cathartic for me. And my friend just held me at the end. He was also there to let people who stumbled upon us know that I was not insane just dealing with some issues.
IT REALLY IS OK TO STUMBLE. NONE OF US ARE PERFECT.