CHEESE: Summon both phantoms, run straight to the boss and pray you'll get there with ~8 estus and enough lifegems. Then, alt+f4 so you can quit quickly. Open the game again, you should spawn in front of the boss room with no summons. You should be in the clear right now (except that ******* iron warrior who can sometimes come and pancake you), so you can buff yourself up if you want to.
Now quit the game and back up your save file.
Open the game again, and give an attempt at smelter. If you die, alt+f4 and replace your current file with the backup you just made.
When you load up the game again, you should be still in front of the boss gate, and now you can do the boss again without having to go through hell.
If you have a brightbug you can use them here. You won't be able to waste them since you're always going to loop back to the save before you use it, if you die.
I know this is scummy, but the devs were kinda scummy too when they made this area, so in the end its all fair play.
If you wanna know about making backup saves, just ask google.
Oh and don't you ****ing dare 'git gud' me, i ****ing *****slapped sinh ivory king and fume first try without summons so i think i'm 'gud' already.