Zee,

That Honda fill plug was rounding off as well. I used the pipe wrench on the bolt head ... got the jaws locked on to the remaining sholders of the hex and broke it free. It chewed the bolt head up but once it got a bite the harder I torqued on it the stronger the jaws gripped the bolt. It may seem strange to use a plumbing tool for auto work ... but in this case I can't think of anything else on hand that would have done the job. It not only gained a lot of grip on a bad bolt head but at 18" the pipe wrench provided a great deal of leverage ... like a mini-breaker bar. I do hope you get this problem resolved through whatever means. Good Luck.