If there are par2 files, grab the first one and run it against what you've downloaded. That'll tell you how many more blocks you need in total to repair it. Then just get the required number of blocks (par1+something, par2+something). The "something" is the number of blocks that file will repair. That way you don't have to grab all the par2 files if you only need 1 block.

Once you've repaired it, you should be able to extract the files without any errors. I use QuickPar for checking par and par2 files against rars.