Banpei's new TA60 family cruiser

[Bean]

One thing you could try is lifting the rear in a way to relieve the tension from the springs as much as possible before you try to undo the bolt. Even if you can't reach the shaft to hold it in place some hits with an impact wrench might help.

Last but not least and a bit more drastic. IF don't get enough leverage for the screw driver (i.e. it's slipping) you could use a dremmel with a small thin cutting edge to make the slot slightly deeper.

For the last part you would need power unless you have a battery powered dremmel.