Good on you for sticking around for the challenge.
Dark Souls 3 optional boss: how to beat Old Demon King
Having taken down Horace, we'll make a run on an optional boss. Return to the lake and explore in the other direction - away from the ballista (which can still hit you, unfortunately).
At the other end is a giant worm. It shoots lightning and damages you whenever it moves, so you need to be careful about rolling away after a few hits, and being able to get out of range of its magic. It’s not that this thing is so hard to kill; it’s that it’s so hard to kill with the freaking ballista smashing away at you. You need to know where the nearest cover is and try to keep it between you and the ballista. It's much easier if you don't lock on.
If you and can’t master the timing required, you can run past it and tuck behind some rocks to the left (facing direction of travel) where the ballista can’t reach you – and neither can the worm. Arrows at the ready! It takes forever, unfortunately.
Update: Depending on where you position yourself, the ballista will kill the worm for you. Try a few locations until you get the angle right. This will also open a second path to the next optional area, but if you take it be aware that the walkthrough won't make a lot of sense.
You’ll receive the Lightning Stake spell and an Undead Bone Shard, and can rush off to fight the boss ahead. When the worm is down it's much easier to collect the treasures around here; again, most of them are Titanite Shards but there's the Shield of Want, too; this has a pretty high strength requirement but increases souls gain from enemy kills.
You can choose to ignore the giant worm, too. The easiest way past is to run to its left, where its body touches the rocky wall. Hit it a few times and then roll away so you’re not damaged by its movement as it raises its body. Just run underneath it while it's moving and sprint for the boss gate ahead.
Old Demon King boss guide
Through the gate, you have a moment to collect yourself before approaching the body in the middle of the room to kick off the boss fight. You may want to pause outside as you'll often find summon signs here, including a mage NPC. Just inside the arena, you can summon Knight Slayer Tsorig if you defeated him back in the Catacombs of Carthus. When you're ready, move to the centre of the room to kick off the battle (note that any players you've summoned can't enter the mist gate till you do this, so don't leave them exposed to the ballista and giant worm).
This is a drawn-out battle, as the boss has a huge health pool. As well as all the usual swipes and slams you’d expect from that mace, which of course come with area of effect flames, the boss has multiple fire magic attacks. Most of these attacks are easy to dodge, but they have really long range and can be difficult to avoid at close quarters.
In combination with the two mounds of bodies in the arena, this makes the battle much easier for mages and ranged players than melee types. What you need to do is lure Old Demon King. Get him to chase you round a mound, and when he commits to an attack, rush round and hit from the other side. Make sure you have enough stamina to get away before he retaliates.
Ranged and magic characters can do this in figure eights around the two mounds rather than just one, but instead of attacking when Old Demon King does they should use the time to sprint to the opposite mound. Then, hit the boss with attacks as he runs towards you. When he approaches, start circling the mound, waiting for your opportunity to gain some serious distance when he attacks again.
In the second phase of the battle, triggered at half way through the health bar, Old Demon King becomes a little more aggressive. He also gains a new spell, which fires multiple large, tracking fireballs. You have an excellent opportunity to damage him during this attack, but be careful; if one of the fireballs hits you, it will knock you down – far more deadly than the significant damage it dishes out, as it leaves you exposed to further attacks and lets Old Demon King get good and close.
There’s actually a mysterious third phase when Old Demon King gets very low on health, where he’ll kneel. If you stay at range and keep pinging him, he won’t get up for the rest of the battle. Strange, but a welcome respite.
When the battle is over, you’ll receive Soul of Old Demon King, and can activate the Old Demon King bonfire.