weld a big hex nut to a rounded head bolt to remove it, if you can't get a pipe wrench on it.
also you can file or grind flats in it for a n adjustable wrench.
I like to use a good pair of Irwin Vise Grips for rounded off bolts. I try to avoid welding on or around cast iron and nicely painted metals so I don't burn the paint off. The filing or grinding works good when there is lots of space to get the tools in. I like to grind the flats to fit a open end wrench because I have found adjustable's to have play in the jaw and loosen off, slipping making the bolt head rounded again.