Another hint: use a line wrench in place of a regular open end. (wrench with an opening, but grabs more than just 2 flats? Do you understand what I mean? used on hydraulic hose fittings normally.) It seems to be harder to round off the fitting this way. At least if this is the same fitting as on the accumulators in my 560SEL. Gives you more "grab" before it strips, it always seemed to me.
I posted a long, drawn out story about doing the cells on my car a while ago. Search under my name if you have some time to read my ranting.
1989 560SEL (172k)
1989 325IC (122K)
2004 Suzuki Volusia LE (3500 miles)
2005 Yamaha Road Star (20 K miles)