The 'Souls series has a storied collection of intense, nerve-shredding boss battles and the first to fall is often one of the most memorable. Dark Souls 3 certainly delivers in this regard, as the slumbering knight Iudex Gundyr presents a suitable initial challenge design to test your freshly 'unkindled' mettle.
Iudex is found kneeling in the centre of an open courtyard that concludes your brief excursion in the Cemetery of Ash. With the safe haven of the Firelink Shrine hub beyond, it is time to pull out the Coiled Sword from his chest and begin Dark Souls 3's first true dance with death.
In comparison to the punishing brutes found in the opening stages of some of the previous Souls games (hello, Asylum Demon), Iudex's first form is fairly simple to conquer. That being said, newcomers and rusty veterans will be forced to become closely acquainted with the game's faster-paced dodge-and-roll mechanics out of necessity during this brawl.
Melee build players should avoid the understandable urge to hide behind a shield, as Gundyr's range of halberd swings will obliterate your puny stamina bar in seconds. Invulnerability frames granted by dodging are quite forgiving in Dark Souls 3, so use this to your advantage. If you are feeling particularly brave you can try to parry his swings during this first phase, but be careful as a few mistimed shield swings and you'll be back at the last bonfire in a blink.
Try to remain at enough distance so that the halberd's point is inches away from you when Iudex attacks, this way you can take advantage while he regains his poise. If his blade gets stuck in the floor, do not approach. He may look vulnerable in this state, but Iudex will happily skewer you from below if you attempt to get a few ill-judged hits in. He also has a powerful jumping attack if you get too far away (not shown in the video), which can be avoided with a sideways dodge.
Remain patient, take your opportunities when you can and keep enough distance to allow for an Estus Flask or two when needed.
After transforming into a gargantuan part-man, part-slug abomination, you will discover that Iudex's attacks are far more frenzied and have a much broader reach. Distance is no longer your friend at this stage, as your hideous adversary will either splat you with his tail, or smack you around with his long, newly fashioned left arm and claw.
When the boss begins the transformation you will have several precious seconds to either take an Estus swig or get in a few crucial attacks. If your health is already low then go for the former, as this next part gives minimal health refill opportunities.
Iudex is actually weak to fire in this form, so if you happened to pick up the Firebombs before the fight, then chucking a few burning bundles at the horrific beast is a valid strategy. This can leave you open to getting crushed while locked into a 'throw' animation. Equally, magic users (particularly pyromancers) will need to be careful with spamming attacks due to the increased attack range.
Those stuck with a trusty sword initially seem at a disadvantage as there is no real safe spot, but with an aggressive approach this second half is much simpler than it first appears:
First, unlock the camera. Things are about to get a bit hectic and your camera focus will be the least of your worries. Once that's out of the way get as close as possible to Iudex's side with the protruding arm and dodge past it when he winds up to attack. This should give you enough time to stab at his back a few times and avoid the axe/head combo on his left side.
As much as he flails about, keep the pressure on and stay close to Iudex's back and he will eventually start leaping in the air. This attack is very easy to dodge and his recovery time is incredibly slow so charge a heavy strike or slice to your heart's content until he eventually bites the dust.
Congratulations! You are now in possession of the Coiled Sword and have gained access to the Iudex Gundyr bonfire and the Firelink Shrine that lies beyond.