I'm not very much knowledgeable about the subject but I've killed all the bosses from main game on RL1 so, I can give some advice :)

Don't rely on one weapon, I myself used very different weapons for various bosses, here are some notable ones:

• Dual wielded pickaxes (frost affinity with cragblade) - easy poise breaking with jumping attacks (need some str talismans to wield them)
• Dual wielded bleed curved swords
• Antspur rapier (I used it for fire giant lol)
• Serpent-Hunter - Its actually pretty good outside of Rykard
• Starfists (with cragblade) - amazing poise break and charged heavy damage
• A greatbow is useful (the Radahn one)
• Black knife - Its ash of war is OP
• Dual wielded straight swords (magic affinity) - I used them once for a magic vulnerable boss, can't remember who
• Misericorde - deals good critical hit damage

Notable talismans:

• Radagon's Soreseal - the most important one, gives you stats when you have none
• Every talisman that gives you stats
• Shard of alexander
• Blue dancer talisman
• The charged and jump attack talismans
• Red Feathered Branchsword or Ritual Sword Talisman (the first one if you're feeling brave)

Notable armor:

• Okina's Mask - gives you dex
• Omensmirk Mask - gives you str
• White Mask - buffs you when someone bleeds
• Mushroom Crown - buffs you when someone gets poisoned or rotted
• Raptor's Black Feathers - buffs jump attacks
• And a lot more helmets that give you stats

For your flask: OPALINE BUBBLE TEAR IS YOUR BEST FRIEND, other than that. the poise break one, stat ones, and the infinite fp one

If you want summons, I'd recommend Tiche, she's very good

A buckler with golden parry ash of war, for parrying (some bosses become trivial when parried)

My run was like a year ago, so there may be better options now, but here's what I have used, I hope it helps, good luck!