1: Sack the painter
2: If softer options have failed then two options from me: -
a) Paint Stripper, apply, dwell then wipe away. Wear proper PPE. This won't take as long as you may think, but will hardly be quick. Stripper quite pricey so allow for that.
b) Steam and a plastic scraper, then turps and a rag to finish.
This assumes this is Gloss.
If tiles are porous then the job will take a whole lot longer. Time thge paint has been down is also a factor.
Last thing, have you carried out a small test patch? If not do so. We have this all the time, though no where near this covered via builders cleans. Longest I've spent was 4 hours and that was a large kitchen diner with paint all around the edges and liberally splattered throughout.