Ceaseless Discharge
HP: 4200
Skills: Tentacle pierce, Tentacle slam, Fire tentacle*, Swipe
Souls: 20000

Difficulty: 1/5 (Running), 2/5 (Fighting)
Times died on 1st playthrough: 1

There's not much left to do actually. Head to the Demon Ruins to partake in the final main segment of your journey, to kill Izalith, who has the final lord soul. Of course, the Demon Ruins/Lost Izalith segment is one of the biggest areas in the game, so there's still quite a ways to go. However, upon entering the Demon Ruins, the entire ground is stained in lava, making it impossible to pass. Thus, the only way to clear the lava - to kill the source of it. Meet, the Ceaseless Discharge, one of the 3 bosses of the Demon Ruins.

He's huge but that shouldn't mean you should be afraid of him.

Ironically, he's one of the easiest bosses in the game. You can practically defeat him without hitting him and fighting him up close. And, even if you choose to fight him, he telegraphs his moves so heavily that its easy to avoid most of them. He has some of the biggest damage spikes in the game, but no matter how powerful his moves are, they are all for naught if you are able to block or dodge them, which should be fairly easy.

Tentacle Pierce - He pierces the area in front of him with some tentacles. Easily avoided by rolling. Does moderate damage compared to other attacks.

Tentacle Slam - He slams a giant tentacle in front of him, leaving it there for you to hit. Does major damage, but easily blocked/dodged.

Fire Tentacle* - The exact same move as Tentacle slam, but leaves a burning cone of fire that does major damage. The additional fire spread can make it harder to dodge, and blocking it is suicidal without a power fire resistant shield. However, he won't use this move at all if you aren't too far away from him.

Swipe - He sweeps an entire area in front of him with a tentacle. Easily blocked.

First off all, if you want to kill this boss like everybody else, it takes almost no effort to do so. Un-equip all heavy armor and equip something that allows you to fast roll. Go pick up the Golden Hemmed Black Set to trigger the boss fight, then its time to run for your life. Sprint like your life depends on it, all the way back to the beginning of the fog gate. He'll probably hit you once because you're too focused on running and didn't dodge, but it  probably won't kill you. The moment you wake up, run again, he'll chase you. When you reach the bridge at the start of the area, he'll hang fall to his death, and will try to cling onto the cliff with his tentacle. Chop it until he falls, you win. 

That's a sure-fire easy way to kill the boss without any challenge at all, but if you want to fight him like a man, here's the run down. The only time you can hit him is through his Tentacle Slam move, which he uses pretty frequently. Just stand in front of him and 75% of the time, he'll use that move. Dodge it, and hit the tentacle to damage him. Rinse and repeat until he dies. If he does a different move, dodge it, wait for his next move, make sure its Tentacle slam, because you can only damage him if he uses that move.

Look here you big f**k

At all times, make sure you stand near him, he'll use any of his moves except Fire Tentacle, which is his most deadly move. It does major damage and you'll likely not be able to block it, the timing for dodging it is also more strict because of the fire damage spread. He only uses this moves if you stand at a moderate or far distance away from him, and will probably not use it if you just stay close to him. Just make him use moves, and punish him during Tentacle slam. He has quite a bit of HP, but you'll take him down easily at this point of the game.

And...that's it for this boss! He gives 20000 souls, not a bad amount for someone so easy. Unfortunately he doesn't provide much else, but at least you can proceed deeper into the ruins now. Onward! Oh, and I won't cover Demon firesage, as he's the EXACT SAME THING as Stray Demon.