I've just started a new playthrough (did not finish my first, just restarted to get a main character I was happier with), and I can't seem to get into the Friendly Arms Inn. The guy that assaults you outside it is completely invincible.

I can't fight him - that will get me killed instantly.
I can hide in the temple next door, but that has little effect as they guards will ignore the guy as long as he's not attacking me.
Going out and then back in rapidly seems to help, but as soon as he starts casting Magic Missiles I can't make it back in. Even standing right by the door, I get hit before I make it back in, and hit = instant death.

Any tips on how to get past this guy?
Didn't seem to be a problem the first time, but then again I had Xzar and Montaron with me that time. Now I just have Imoen (cause the two others were being dickweeds - possibly due to my low charisma)

What type of character are you playing?

I'm usually a mage, so tend to use buffing spells to prevent him from killing me quickly.

Different character classes will have different methods of dealing with people.

You also need to make sure you are as well armed and armoured as you can manage at that early stage of the game (Doing all the quests at candlekeep means you should have enough money to kit your character out well enough from the candlekeep innkeeper).

That is a remarkably hard fight for the early part of the game. My best suggestion would be to try and get montaron & xzar, even if they're being dicks (but not actively refusing to join) they're good as meat shields if nothing else.

Potions can be handy if you can get something to boost your defence or attack or better yet get immunity to fear, his mirror image spell does make him pretty much invulnerable for a few rounds, its almost impossible to stop him casting it and he almost always casts fear right after which can halve your useable party so anything that can negate that will drastically increase your chances (most every mage assassin in the game uses mirror image & fear as their opening moves so you really need to get used to beating it). You can avoid triggering him if you stick to the southern wall and make your way around to the temple to buy more supplies too.

Whilst its not the best tactic, waiting for the guard patrol to get close before triggering the encounter helps, they often jump into the fight if you can survive the first round or 2, they can help but you don't get the XP if one of them kills him

You could also export your character, restart the game with him/her and get a slight advantage since you'll have the money for better equipment

Start attacking him with missile weapons/spells right before you talk with him. If you time it correctly, the attacks will hit just as he starts to cast mirror image. He'll fail the spell, and then he isn't that scary anymore :) (sort of cheap, but it works). Also, it really helps to fully explore the areas on your way to the inn, an extra level really does take the edge off.

I find that the Wand of Magic Missile Imoen comes with immensely helpful here, as it's a guaranteed hit. Still, it's a luck based mission overall. If you can interrupt his Mirror Image, he's a pushover. If he gets that off but your party resists Horror, you've still got a shot.

Fight him when as many guards as possible are nearby, they will help you a lot. :)

EDIT: Oh, i see this has been mentioned before... :(
But it's a good advice anyway... :P

haha, yes. Must be the only place in the game where that wand is of any use :P

I like to put the party just below the steps, so his script doesn't trigger, then sneak Imoen behind him and force attack him with the wand of magic missiles.

Let that get off and he'll turn hostile. Immediately pause and move your party into position throwing everything ranged you've got at him, while moving your melee units in for the kill if he's not already dead by now.

With a little luck he'll be dead before he knew what hit him, or very shortly thereafter. Surprisingly this trick doesn't even alert the guards.

Edit:: I see this is pretty much LordCinnamon's method as well.

This guy is way too hard early on, along with some of the other assassins. Aside from using spells or cheap tactics, you can just go elsewhere and level up once or twice, which will make the fight much easier.

I remember the first time I played, and I'd started as a mage. I decided to do all the Candlekeep stuff first, so I went around all the buildings.

My big mistake? Not resting after I met the first assassin. I didn't think there would be a second, and I was all out of spells... My only save was the auto (upon entering the building) so had to reload until I could kill him in melee, which is damned hard.

I'm playing a Cleric. I got past the guy now, but these mage assassins seem to pop up quite often.
Will avoiding them until I get a few more levels make it easier, or should I just try and stock up on potions/scrolls that will counter their spells?

More levels always helps, but you need to have a good strategy too.

You need spells/scrolls to break their enchantments, and Clerics have some good ones.

Also, if you can get a fighter/thief into melee with them, that helps a lot. You can interrupt spell casting that way, and mages have low health too.

What you need is buffing spells, to combat fear and to get hit advantages.

The best guide I've seen is here:

http://www.sorcerers.net/Games/BG2/SpellsReference/Main.htm

Click on "Divine Spells" on the left hand menu.

It gives you an idea of the usefulness of each spell, as some sound better than they are.

Having a friendly mage helps as well - it's always fun to use their own strategies against them....

Actually, that wand is quite awesome for interrupting spells and, subsequently, defeating otherwise very hard mages in the beginning.

Pick up the evil NPCs on the track west of Candlekeep. They'll give you the extra firepower you need.