I am stuck at  the point where I have lost all weapons and am using the
crosshairs as a defence weapon

Spoilers
.

Spoilers

I have just Killed a Large antlion and now go down some stairs, at the
bottom the force field doors open when I apply the crosshairs,I enter this
long high room where the left and right walls are made of Combine metal
materials.
I can go only so far in this room as gravity or forcefields slow me and hold
me back.
A large triangular ship appears and spawns a batch of manhacks, which I
dispose of, this happens repeatedly untill I die.
There seems nothing I can do to go further, There are two areas on the left
and right walls that spark when  aim the crooshairs at them, I assume this
is part of the answer but I can't reach those areas or affect them any other
way.

I am really enjoying this very good mod.

Regards, Rene