To undo revisions 118 and 120:
svn up -r HEAD # get latest revision
svn merge -c -120 . # undo revision 120
svn merge -c -118 . # undo revision 118
svn commit # after solving problems (if any)
Also see the description in Undoing changes.
Note the minus in the -c -120 argument. The -c (or --change) switch is supported since Subversion 1.4, older versions can use -r 120:119.