Im playing ekko for a long time now and i tryed all the build finally i wound the best 1 hard core carrying in the late.

Start whit the dark seal and reffilable potion
Frst back: 750g needed,upgrade to the coruption potion and get dorans ring(boots) ofc
Start building ROA and get sorcerers boots.
Defence iceborn and spirit visage (depends vs who your playing) ad or ap.these items will give you tank+utility(slow,CDR and +25%healing whit your ult)
Finishing whit majais soulsteler if fed if not start building rabadons deathcap.At this point you will have 400ap,tank,CDR,hp,sustain.Last item is situational.