I'm lv11, doing a minutemen mission where you go to a settlement on the shore, they tell you about Ghoul attacks and so then you go to some Guard Army place to find the ghouls. I've gotten to almost the end, I just need to take out a glowing ghoul but this thing is invincible!

As I said I'm lv11, on me I have a few rounds left in the harded double barrel, a few in a heavey sniper and a few in old faithful - like 5 of each. I got an ok pipe revolver too with 38 bullets left. Can this be done?? It all seems to just tickle him! And the fights indoors in a small building too... I think I have 1 frag also

That is a VERY hard area for 11. I'd bring a friend; the Dogmeat or Preston (if Preston is available). Also in the bunker (building) behind the Guard Station is some Power Armor. But be advised, there are ghouls around it. And if you get it a bad-ass robot will track you down once you leave that building. So be careful.

You could also use your Power Armor. Assuming you have enough mats to repair it. And fusion cores.

But since you are inside already: clear out the place first. Kill all other ghouls. Do you have a mine? If not, you are going to have to kite him around the place firing and running. I won't be easy. If you have a mine, place it at the door, the entry-way to a room with a collapsed ceiling. He was below it in my game. Once mins is placed, fire, then back up, down the hall. When he hits the mine, unload.

The mine is how I killed him. I did die a couple of times. Cause I had to get closer him so he get pissed and chase me up that collapsed ceiling. And he is fast.

On my second play-though, I got the power armor, ran from the robot, and went inside. Wearing power armor he's no trouble.

What difficulty are you on, for reference? Yeah, it's definitely killable. A Glowing version of an enemy is a rarer and stronger than normal version, but nothing special. If you're having a lot of trouble, I would suggest definitely using the frag grenade. Run back and let the ghoul come to you until he aggroes onto your companion. Let your companion tank. Obviously shoot at him while he's going for your companion, but you can just stand and shoot from where you're at without having to kite at that point. Aim for legs if it looks like your companion isn't able to tank for long. That makes it really hard for the ghoul to go after you after it takes out your companion. You just keep kiting backwards and shooting after that. Or run to the left or right if you run out of room.

You might be a bit underleveled but if you can cripple one of it's legs it becomes practically harmless. Attempt a sneak attack with your frag then target most damaged leg in vats early and often till it comes off.

Fat man in the face will work.

Or you can drop the difficulty down to the lowest available to get past the fight makes more sense.

Do you have mines, can you lay a mine trap and draw him to it to soften him up? If you have no companions on you, no Dogmeat to run in and take aggro. I would suggest using trickery to win the fight. Set yourself up on a higher elevation and maybe use the terrain to keep him away from you. I was recently in a light house and fought off and killed 2 Deathclaws by staying on the stairway, and using loud sounds to draw them in while beaming them in the head with sniper fire.

Fighting fair are for people who are dead. Use the environment against it in every way that you can.

I've got no one with me, not even the dog... If I go back outside to grab the armour, when I go back in will those rooms I've cleared still be cleared?

Also, I've unlocked some terminals and found a dead woman with a tape - seems unrelated? But since I'm already there is that worth doing?

With regards to the main quest, all I've done is gone to diamond city and been told a detective is missing. Thought I'd use minutemen quests to lv up first?

What do you mean by "Thought I'd use minutemen quests to lv up first?" ? If you go outside to grab the [power armor, everything you did will stay cleared. Though it's usually better to have waited until later in order to actually go to this location, since you get better armor the higher level you are, where it maxes out when you visit the place around the high 20's. I'm not sure how close you have to be to the armor before what power armor you get is set in stone. Just leaving the building, fast travelling to dogmeat, then fast travelling back should be sufficient. Or you can just grab the power armor you already have. If the question that you mentioned was something along the lines of "Should I have done the Minutemen quests first for xp", then yes, that's a possibility if you want. I don't really remember what a normal level would be at your point in the game, but from what you described, it seems you skipped a lot of stuff and just went directly to Diamond City. You could alternatively also do all the Diamond City quests and all the quests you skipped along the way down.

I'd recommend poking the Brotherhood. Meeting up with Danse in the Cambridge Police Station, after a short questline you unlock two "refreshable" quests that you can go do and turn in at the same time, netting you ~500 xp (Depending on Intelligence stat) and ~200 caps. I tend to grind those on my new playthroughs for a while. And they can be accessed at any time, regardless if you plan on allying with the Brotherhood.

(Though I imagine the quests become unavailable if you betray them later on).

I've skipped stuff?

I came out the vault, went to Sanctuary, rocket, Concord, brought the guys back to Sanctuary and have been doing their/minutemen quests one after the other?

The old woman said go to diamond city for the main quest, so I went there, they said find the detective and so I put the main quest on hold at that point... is that not a very good way of doing things?

And is it something like 3 days when cleared places respawn?

Possibly 3 days, I just know that it's a pretty long time. The game's intention was to let you explore and get sidetracked on your way down to Diamond City. Otherwise, you would be underleveled. There were a lot of different settlements/locations along the way to Diamond City that offered side quests with xp and/or good rewards, most noticeably the side quests that start at the Cambridge Police Station. I would recommend taking your time with the Minutemen quests since there's a lot of them.

For interior locations it's around 20 days unless a radiant quest forces it to respawn. Outside it's 3 days.

Ahh you missed the BoS line. On your way to Diamond City, you should receive an emergency transmission. It starts a quest (forgot the name).You can here it in Lexington.