how to remove one-way screws?

I bought a bungalow, with locks on most windows. I can't remove a
window to repair it, without removing the window lock first.
It has 2 one-way screws fixing it to the window frame. I can't
drill into the screws, as they are a hardened metal. I tried
gluing a screwdriver the the screwhead with epoxy resin, but that
didn't work.  I could attack the lock with an angle grinder, but
I know that will make a mess, and require putting lots of protective
covering in place first.
Any other ideas?