Another Settlement Needs Our Help - I'll mark the location on your map

I had a rather…something experience with one of these quests.
I had to go the National Guard Training Yard in order to kill the ghouls there. I only do these quests because they're rather easy and I gain experience quickly from them. When I get there, I realized I made a mistake because I'm only about level 9 or 10. I have…difficulty with the final Glowing One in the building, and I use most of my fusion cells in the Laser Musket (my character specializes in energy weapons), and I have little ammo left. I remember that I unlocked the armory door so I thought "hey, there should be some more ammo in there" and I went in. Only found some random ammo for guns I didn't have, some adhesive, and a suit of T-45 power armor missing the head and right arm. I get in the power armor because "why not?" and go outside. Outside I'm immediately lit up by a Sentry Bot. Not just any Sentry Bot, a Legendary Sentry Bot. I proceed to spend 30 minutes running around a pile of crates shooting at it and try to not get shot. At this point I have 8 stimpacks left and all the armor pieces are broken. I finally use all my AP to VATS the Sentry Bot in both arms, disabling it. It then runs up to me in the intent to self destruct. I try sprinting but I realize I'm fucked because I used all my AP. I then die from the explosion. This happens about 4 more times before I go "fuck it" and just try to run away.
TL:DR This quest got me raped by a Legendary Sentry Bot.