How to kill the Mega Interplanetary Ninja Assassin Claptrap?
Kill all the turrets first; they don't respawn. They're not too hard to take out (relatively), but they're occasionally covered by invincible covers. Kill the turrets one by one if you're having trouble taking them down before dying; their health does reset but only if they're alive. Make sure all turret slots are dead before preceding. IIRC, corrosive damage helps as well, corrosive sniper would be best, failing that a magnum or SMG.
Stay the heck away from it; if it runs you over it's an instant kill. Just run around at a long distance and shoot it's eye (that is the weakpoint isn't it? Been a while), there's several places you can hide while you're doing this as well, use cover well.
Don't kill the claptraps except the Kamikazes or the ones getting too close/hurting you too much. They're there for second winds.
Just tackled this one myself.
First, make sure to equip a shield that's fire resistant. The asbestos one, with very high resistence, is aces. Fun fact: with that shield, the eye laser does not hurt you at all. It doesn't even bring your shields down. You can shoot the eye at will.
Second, run to the left from the area where you're respawning. At the very far end you'll find some ruins, there's a wall there with a partial window and you can comfortably crouch behind it. The kamikaze clappys won't reach you back there, they will madly run towards the wall, hug it, and explode, with no harm to you. This is useful as they drop stuff, bullets and health.
From behind the wall you can sniper out the turrets and finally shoot the eye at will. At some points the giant claptrap will become very close to you, this is the ideal opportunity to shoot the laser eye without taking damage from the frontal turret.
I hope it helps someone out there.
Actually, this "Fyrestone" map gives you at least one safe spot (I don't need more than that, obviously) where taking down the big toaster gets rather simple. I don't feel like making a video now (tried a screenshot, but I can't find a shot that clearly shows the position), so I'll try to describe it. You can use the original Fyrestone map to follow my directions; the DLC4 version looks pretty much the same.
If you stand where you start the game (after getting dropped of the bus) turn left and go to the southern fence (presumeably the bus CAME through there, although I have no idea why they nailed it shut afterwards). Follow the wall at the east. On your left, you will pass a large stone, then a small stone, then another large one. Then you'll get to the largest stone yet, with a path between that large one and a smaller one. Crouching in that "passage" behind the large rock will prevent the Kamikaze claptraps from reaching you, and the big bad can't do much either unless you stand up to take some shots at him.
Sorry, that's just the easy way of doing it. You can always do it the hard way, running around, taking care of the Kamikaze guys, trying dodge the big guns and nibbling them down. Presumeably that didn't work out for you, though -- playing a Siren, my own boss-combat techniques are of no help to you since you aren't phasewalking no matter how hard you try, so I'm not trying to give you soldier advice :-) My own soldier isn't that far yet, and he won't go open combat since he has a big sister that tells him where it's safe :-)