The text for the smelter demon is false and i mean like completely false!!!!
in scholar of the first sin smelter does nearly no damage to her at all which means it is highly recommended to use this fight for her 3 bossfights as it provides extremly good surviveability. Furthermore she helps you survive the fight too. However it is recommended to use firedefense increasing armor, rings and items or pyromancy to make it much easier for yourself to allow you to do damage. Gyrm shield would be perfect to block all his attacks including his blast. As Smelter demon has a quite easy attackpattern and as he is vulnerable to attacks from behind once he focus on lucathiel for example it is extremly easy to beat him this way.
Lucathiel herself takes only like 4% damage per hit and 5% if the blast hits her even in higher NG+ while she is actually blocking most of his attacks. I never faced a fight with her beeing below 60% health at all.