Like all bikes now, it is the darn stupid Phillips head of the screw. So you need an easy-out to drill in & back it out. Same problem I had only knew my battery was in its last season, frustrated, & such so reason I cut it off & put in a new battery. So take it easy use the proper easy-out & then replace the metric scew with a new one, & if by chance you can buy a SLOT head you will be even better off. Put a bit of lube on the screw before you screw it down & do not apply to much pressure.
You might find the nut below tends to move around so block it with a shiver or wood & in my case I have stopped the nut below from moving with the thick head of a toothpick.