Always use 6-point sockets when you can to get max grip on a fastener and reduce the chances of rounding it. To help break the bolt/nut use a small propane torch to heat the fastener for about a minute, then let cool for about 10 minutes and try to loosen. The different metal components will expand and contract at different rates which helps release them. You may need to repeat the process, but no more than twice.

Just be sure when using the torch to keep the flame on the fastener and you shouldn't have any issues since the surrounding material will absorb the heat. Use common sense when working with an open flame!