you probably need more +% fire spell dmg for Jday. I think key to Jday is fast killspeed (correct me if i'm wrong, potato). if you can kill imperius/malthael before he casts nuke, then they're not a big deal.
your current belt and gloves don't add to spell dmg. they should probably be switched out for fascinating shrine crafts. i assume u know how to shrine craft. go for moar +fire spell dmg. don't forget to shrine bless, as well. prob MO those with fire spell dmg, as well.
ammy should prob be rerolled with +2/3 all skills and as much +fire spell dmg as reasonable. MO'd, for more, ofc. same with ring-- +1 all skills, fire spell dmg. and mo'd. in
oskill doc, nightmare desc says that it causes slow and reduced accuracy to enemies. supernova is prob lightning element. we r not optimized for lightning spll dmg, so both these procs don't help us much.
for jitan, you could try summoning him outside, then killing his guards little by little inside the corridor. run down into the halls of anguish to heal/resummon hoplites, then go back and kill his guards more. rinse and repeat until only jitan is left. from there it's easy.
tbh, i think jitan uber was made to require higher defense from us ~400-500k. if the above doesn't work, you could try respeccing to blood thorn melee. blood thorns counts as fire spell, so +fire spell dmg you gathered for Jday would still benefit you. and maxed tainted blood + terror strike will boost your defense. (i don't know if it'll be enough, i completed it with previous strategy at 200k defense...but it's a possible strategy you could try).
hope this helps