The first time I saw one was in a trailer near Arefu. I was level 3, and due to an unfortunate run in with some radscorpions, I only had about 30 bullets. I saw the thing on the radar, and assumed that it would be a raider, hopefully with lots of ammo for me. I sneaked around to the opening, and it saw me immediately. I had never played a Fallout game before, so I had never seen a Super Mutant. This was also my first experience of coming up against a minigun.
I actually did manage to kill it, but the only functional weapon that I had left was a knife, plus I used all of my stimpacks. I had to start the game over.