Personal advice on a starting character specs out soemthign liek this (may be off on my stats)
ST 4
PE 8
EN 2
CA 4
IN 10
AG 10
LK 4
I think those add up right but use gifted and small frame to get an extra 8 points to spend, tag small guns, speech and energy weapons. This is what I spec out quite often, run up small guns to 100-120% before taking speech up to 100% and then energy weapons once you start on to the necropolis.
I'm somewhat of a compeltionist so my games run 80 odd hours I think.
As far as the death claw goes you need to do around 160-180 damage to kill one. general strat is take an aimed shot to teh eyes adn run as far as you can, rinse repeat. and it's sometimes easier to do solo because yoru NPC's just stand there and get one swiped.
If you can wait until after you finish necropolis and get the plasma rifle, which will make really short work of the deathclaw. Also note that with 10 AG you quite often get 2 turns to process before the NPC gets 1 under the conditions that the NPC initiates combat. There are some exceptions to this, but for the most part it holds true for teh first half of the game.

Don't start a fight, walk in and confront Garl without initiating violence. YOu will mroe often than not be allowed to get within talking range and like most of the rest of the game you have the option for violence, compensation, intimidation or persuassion.
I have succeeded in releasing Tandi in the following ways:
Dueling Garl in hand to hand
Offering cold hard cash (200 caps I think)
Intimidation
Persuasion
and Obliteration
And if you really want to catch a running joke show up in leather armour and dogmeat and no other NPC's

I didn't have the shotgun, I actually got the shotgun from the Raider building. I got the grenades and SMG from Vault 15.
I went through at least 5 or 6 Stimpaks killing Garl and the other goons at the start of the battle.
At the start of the battle, I appeared about 5-7 hexes above the exit grid, with Raiders on all sides of me. Everyone starts attacking immediately. Some were quite close, only a few steps away.
Garl was not in the building, he was standing near the closest tent (up and to the right from me). The other raider with a name was a bit to his right (my left). The two of them shot at me with .44's while everyone else stuck to melee.
Does Garl usually start in the building when you try the mission?
My experience may have been different because in my previous visit to the Raider camp, I shot someone in the face and stole his stuff. What can I say? That's just how we Vault 13ers roll, baby! He was giving me lip, and then he tried to shank me. He totally had it coming.
Anyway, attacking a raider before I received the Tandi mission may have pulled Garl away from the building and prevented the execution. By the time I got to the building after killing and looting Garl, Tandi was alone in a locked room. There were only a few Raiders left that were still willing to fight. But they were really spread out across the map and came at me one at a time.

That would do it.

For rescuing Tandi, I just slowly murdered each person there. I'd take out groups of two or three people at a time, then leave and go back to shady sands to heal, then I'd go back to take out a couple more. Eventually I would have to take out maybe four or five at a time, but if you've already cleared out Vault 15 (possible by bartering a rope off of seth and some different people in junktown, or they're lying around places, I forget where exactly), advised the farmers about crop rotation, and cleared out the radscorpions, I find that I've got just enough gusto to take 'em out.
One of the mods I recommend is the NPC mod. It allows for Ian, Katja and Tyho to gain up levels (1 per players 4), change armor and combat tactics.
and Dogmeat can be ordered to wait.
But you need the 1.3 unofficial patch to use it.

If you have good intellect and Charisma, you can talk the leader of the raiders into releasing Tandi. Of course after I did that and returned with her, I decided to go back and take them all out any way... I did that by staying towards the outskirts of the camp and drawing them to me. By doing this I also freed the two slaves they had taken.

I played through this a second time with Ian along, and it was a total bloodbath!
On my first visit to the camp (before the Raiders kidnap Tandi), I killed two or three of the Raiders in the tents instead of the guy at the door.
When I popped onto the map with Ian, I wasn't completely surrounded like my first try. With the Raiders a little more spread out, it wasn't hard to start mowing them down.
But the OP is right, it's really tough to keep up with Ian because he starts running full speed all over the place. That guy can fracking move! I have a 10 Agility, but I think Ian's Agility must be a 10 as well. He usually shoots twice every round, which means10 APs, or maybe Fast Shot Trait. Maybe he also has some king of movement bonus.
Anyway, once the first 5 or 6 Raiders were down (including Garl), Ian fracking bolts into the building. He doesn't even give me time to loot the bodies, the idiot! I'm trying to catch up with him while occasionlly getting distracted by the need to pump bursts of SMG fire through Raiders.
Eventually, I guess wrong about which way he's headed and he gets a few rooms away from me, then he gets knocked down with a Critical right before I'm within range. I took the Awareness Perk, so I know that he's only got 1 hit point left, the stupid bastard.
I have a shot at the guy standing over him, but unfortunately, one of the Slave Girls is standing right near the Raider. Oh well, nothing for it, I open up with the SMG. Mingled chunks of Slave Girl and Raider spray across the room. No one messes with my fracking pack mule! I eventually track down Ian hiding in a corner of the map and pump him full of stimpaks, bringing him back from the brink of death. It's a good thing, too, because not one Raider managed to run away and I had a butt-load of loot to carry away.

NPC Mod for the win!

I tried to talk to him but I get two options, one where I say I want you to release Tandi and he responds by shooting me, and the other where I say I want to, I think, kill him,and he shoots me.

your charisma has to be up high in order to talk him out of it, as well as your speech skills
EDIT:
Never SOFT always HARD

Thanks.
This is why I take lots of guides with a grain of salt, and download multiple diffferent ones.