The damage may have already been done, and the square crank 'hole' (I am sorry, I cannot think of the term for it), and will no longer sit tight on the crank axel because it has now become more rounded than square. If that is the case, it will always move from now on... thereby causing more wear. Your crank axel will have also suffered some wear from this movement... but it is harder than the crank, so it wll be less.
But try this... and hope.
Grease both the axel and crank... grease the bolt thread, and then go for broke when tightening them up. I mean really lean on the wrench. You can do this because you have greased everything.
This a new trick I recently learned from the good members of this forum!