What's the best way to remove paint from a concrete hollow tile wall.

Use Citristrip a safe non-toxic paint remover

Try pressure washing it.

A non-chemical option would be to try and powerwash it off. I would try that first, if possible. There are chemical stripping agents out there designed for concrete pain removal. They are can be caustic, so check with your local hardware/home improvement store expert for specifics of use and ventilation, etc. Gasoline works, but EXTREME caution must be utilized before, during and after use. Make sure that the area is well ventilated, free of cigarette lighters, matches, grills, fire pit, etc. Once the area is cleaned, it will then need to be washed with soap and water.