The little guy bugged and went invincible for me when he walked into a gas cloud after one of those poisonmummies modified from heretic. Not sure if this has been reported yet since I seem to remember seeing a thread of him getting bugged. It was actually on normal doom 2 though so Im sure it wasnt in some modified wad I usually play.
Btw Ive soon finished vanilla doom 2 on uv using only the chainsaw and items, its suprisingly easy and fun. Gibs everywhere.