Since we have regular runs covered I'll try to cover a SL1 no upgrade run difficulty. I only have Nameless King and Soul of Cinders left, but I have a feeling they will both be up topping the list so I'll put them there for now.
1.
Soul of Cinder - Insta-death combos on SL1 especially in phase 2. Hard to find openings in anything other than spear and sword forms in stage one. A long battle you are very likely to mess up.
2.
Nameless King - Phase one is okay. You have to fight the camera, but the King of Storms goes down easily enough with a buffed Dragonslayer's Axe. Good luck with part two though. Tiny chip damage with no real alternative than to get in there. Some attacks are hard to dodge perfectly.
3.
Dragonslayer Armour - MY GOD. Before this fight was done, I was ready to rip my hair out. On a SL1 no upgrade run, you don't have the ability to access any weapons that do any kind of respectable damage to him. You chip him down, hoping you can dodge the quick shield bashes that take off half your life, only to reach phase two. This is where it gets infuriating. Two of his two handed attacks in this phase look very similar, and the shockwave comes out too fast for you to roll after it starts. Prediction is a big part of this fight. In addition, his attacks can potentially kill you in one hit unembered now.
But most infuriating is the fire from the Butterflies. I had to turn down the epic music just so I could hear when they were firing. The laser was easy to dodge, and the mini fireball storm was annoying, but didn't kill me. But those large shots kill in one hit, and you can't dodge through them. Even with the Carthus ring I couldn't roll through it. If you are not ACTIVELY running when it reaches the bridge, you're dead. If he hits you or makes you start dodging, you're likely dead. It's crazy. All of this is going on while you're still dealing miniscule chip damage. Biggest pain in the ass so far.
4.
Dancer of the Boreal Valley - Same problems as the one before: You just simply don't have a viable damage option for the fight. Any spin move the dancer does kills you in one hit as well. Putting you in a battle of attrition that it will almost always win. In phase two, you have to resort to hit and run, as just about any hit will kill you in one shot. Punish and hope for the best in this long, drawn-out battle.
5.
Lothric Princes - Here you have a weapon that can actually be buffed to deal end-game damage reliably on Lorian's weakness. The problem comes in the fact that you have to kill Lorian, who has many moves that will one hit KO you at least three times. Once you have the first part down, it's pretty easy to get him down once, but Lothric jumping in makes it a pain. You have to hide in a narrow hallway behind pillars and fight not only the princes but the camera as well as you dodge Lorian's attacks and take cover from the homing barrage. Getting slashes in on Lothric when you can helps out a lot, but it is a very precise and demanding fight.
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6.
Deacons of the Deep - Not even joking. They were a massive roadblock on this run. On a no upgrade run, you are not at all equipped to deal the kind of damage necessary to take these guys down. In phase two they get so aggressive and kill you in so few hits, you may never hit the Archdeacon after the initial backstab. Then the bastards try to heal on you negating your damage. All the while, fire and wrath of the gods are raining down on you. Then you have the curse to worry about... This is a totally different fight in a SL1 no upgrade run, I recommend everyone try it before simply laughing at how easy it is all the time.
7.
Abyss Watchers - Another massive roadblock, these guys just won't let up. You can parry them for decent damage, but trying to deal with multiple watchers is madness. And even when the third one hops in, good luck trying to deal some damage safely with melee.
Part two is infuriating (and immolating). You can't parry anymore, and he becomes much more aggressive. The fire sweeps will easily kill you in one hit, and your only safe opportunities to attack arise through dodging aoe attacks that kill you in one hit. Sometimes you don't quite make it out of the range, either.
Damage output is poor but manageable for these guys.
8.
Oceiros - Oof, this was a rough one. Done before DSA I didn't have access to my Dragonslayer Axe/Gold Resin combo that would have likely made this fight rather easy. Phase one I was doing about 90 dmg per swing of a lightning infused Battle Axe. He doesn't give much chance to buff, so I just went with that. His leap attack kills you in one hit, and will get you more often than you would like at first. He constantly backs away from you and does those annoying quick swipes. And his breath attack kills you quickly as well. If you can get behind him, phase one is easy.
Phase two is the rough one. Getting behind him lets you get some swipes in, but you have to dodge his spin attack constantly. He also loves to dash around and leap across the stage, making you pursue him and remain very aggressive, which is where we tend to get sloppy. If you are underneath and hit charges, you die. If he charges and you misread *Easy to do* You die. You will die to his charge attack multiple times. If you can avoid that you may make it through.
9.
Aldrich of the Deep - I had quite a few problems with this fight. His Soul Spear attacks are no problem, but in general, letting him do magic is a bad idea. His swipes kill you in one hit, however. And they appear to have variable timing to some extent as well, making this an easy fight to mess up. If he starts to spread those homing shots in the air, don't even try to attack him, just keep running. 3 of them will kill you. Arrows are no problem in this phase.
Phase two: Good luck. He will spend 5-7 seconds straight spreading the homing missles. If you are lucky you will be able to dodge them without immediately having to dodge the arrow rain when you are out of stamina. Otherwise it's instant death. The spin attack is easy to dodge but has deceptive reach. And that fire trail knocks off your health a lot faster than it should. As difficult as it was though, the extra practice through Anri's Quest (Shouldn't be dead, since you can't do Yoel/Yuria's quest line) helped a lot.
10.
Pontiff Sulyvahn - What an intense fight. On a SL1 run, parrying is the path of least resistance. Figuring out what attacks (His fire sword) you can parry, and when to do so is most of this fight.
Phase two, you need to do two charge attacks at the right moment when he is summoning his stand. If you do it correctly, it may die right there, making phase two easier. Then you can parry the same attacks and rinse/repeat. Just watch out for the fire AOE's and retreating slash attacks. If you miss the easy kill on the stand, good luck. This fight becomes much harder with the clone out, and you are much more likely to die, kill the clone before anything else.
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11.
Champion Gundyr - Just like with Pontiff, parrying is the way to go. He's weak to Lightning, so that Dragonslayer Axe is the best weapon again. Parry the right attacks, and you should be untouchable in phase one. Make sure to use a couple of parries to buff/heal before phase two starts.
Phase two, you can parry his charging attack as well as the safe ones from phase one. His shoulder check is a huge problem and can OHKO you, so when you aren't able to parry and he is closing in, just dodge, dodge, dodge. Knowing this now, I can't see him giving me any problems in the future.
Vid
12.
Crystal Sage - A rather simple boss in a regular run, this turns into a literal bullet hell on a SL1 run. His slow homing attacks kill in one hit, sword kills in two (but the homing attack will have looped back around by then and will kill you anyway), and his other magic attacks will also kill in one hit. Phase one is okay if you've gone through it before. But phase 2? *Shudders* Don't try to take out the clones, Aim for him directly. They will all go away when he hides underground again. If you're lucky, you can knock his health down without them even firing in phase two. If they start, you are likely dead.
13.
Old Demon King - Did this last night. It wasn't too bad, you can dodge through most of his phase one attacks easy, but the AOE's are a bitch. The fire spit move killed me far too many times. Once you can recognize the tells for it, it's totally doable.
Phase two isn't much harder. You have to watch the ring of fire to know where it's coming from when it's summoned, as it kills you in one hit. The meteor rain shouldn't hit you though. Just watch for the explosion he does there at the end of the fight. That killed me once, and it was more than a bit upsetting.
Vid
14.
Curse Rotted Greatwood - Unlike in a regular run, the mob in phase one is more than an annoyance, and can kill or combo you easily in this fight. Especially the Red Eye worker and the Hammer worker. In addition, it may be harder to get the pockets to burst due to damage output.
Phase two isn't much harder, but you have to get good at dodging the grab. It is a OHKO in SL1, and it will end a run very quickly. Other than that, this fight wasn't too bad.
15.
Ancient Wyvern - No harder than a regular speedrun really. The Chain Axe knight right before the ladder can kill you easily if he follows you to it when you're climbing up, but otherwise, you shouldn't have too many problems here.
16.
High Lord Wolnir - Killed me once, because he sent that fog out really early. Other than that it wasn't any different from regular NG.
17.
Yorhm the Giant - I had Siegward with me this time (For once) and that made this fight a bit of a joke. I bet that with the damage Yorhm does, this would be a rather hard fight without him in SL1.
18.
Iudex Gundyr - Not really that hard, just dodge the shadow's attacks and it shouldn't be any harder than your first time through.
19.
Vordt of the Boreal Valley - A complete joke with a fire reinforced club.
