D
Dumbo
I was thinking . "You best get the builder back to sort his shoddy work out , I will also leave this yellow stick which has a bubble in it. "Albert's idea would work but is very very labour intensive.
Grinding with dust control would be physically easier.
Whatever you do it's going to be an expensive soul destroying pig of a job.
Personally, I'd offer suggestions to the customer on how it could be done, and offer to come back when it is done!