I recently had to go through a similar experience to replace my clutch. Those 4 **** bolts on the cat converter were not my friends! I took my time thankfully and religously soaked them in WD-40 for a period of 5 days.
When I finally got the toughest one loose (didn't break any - yay!) I had to position myself in such a manner that I was practically doing pull-ups on the **** bolt! I don't have any power tools unfotunately. Maybe it was a good thing - didn't break anything.
You can pick up kits at Sears in the Tool section that are made specifically for rounded off bolts. These kits are great! If you are not going to do it professionally I would recommend hitting the bolts over a period of days with rust inhibitor (WD-40) then eventually using one of these tools to remove the bolts.