I've done this on my 98 T5M. I used a very small wrench that
completly surrounds the nut (not a box-end wrench). I think it was
from a set of bicycle wrenches. It was very difficult to open but I
don't think I had as much problem as you are having.

It was also difficult to bleed. First time, I got a bit of air in
the system and the clutch pedal practically fell to the floor and
would not dis-engage the clutch. Second time I was more careful
with closing the valve while fluid flowed out.

I used a power-bleeder for this from:

http://www.motiveproducts.com/

I bled it because I find the clutch difficult to engage smoothly. It
made no difference in the end.

Volvo has a TSB for servicing the clutch for those cars that have
difficult gear engagement (98 model year) and the TSB shows some
special tools. The tools actually look more difficult then just
bleeding it with a hose.

There is also a newer clutch pedal return spring that changes the clutch
take-up characteristics (installs in the car on the pedla). I haven't
tried the new spring and wonder if anyone else has?