Repot, getting the old soil out & off the roots as much as possible. Ruthlessly remove root areas feeling/looking spongy, slimy, or rotten. Put some gravel about 2" deep in the carefully cleaned or even new pot before adding new soil & the plant. Cut back Severely all the dead/dying leaves, they only drain its' remaining life. (And continue to do so as leaves wilt).
In future, water a lot less!