How to loosen build plate levelling screws

[WYGIGS]

Hi,

I need to level my build plate, but I'm having trouble with the hex screws. I've been able to loosen the front hex screw, but I'm unable to loosen the side hex screw - the hex wrench just slips, and I'm concerned if I try much more I'll damage the hex slot too much. Any ideas how to loosen it? 

What is the smaller hex screw at the back used for?

I hope someone can help

WYGIGS