www.duas.org has many for this. 'La hawla wa la quwwata 'illa billah' x 100 every day is good. There is also a powerful du'a from this website, as follows:
يَا قَاهِرُ يَا قَهَّارُ يَا ذَا الْبَطْشِ الشَّدِيْدِ اَنْتَ الَّذِىْ لاَ يُطَاقُ انْتِقَامَهُ
yaa qaahiru yaa qahhaaru yaa dhal bat’shish shadeedi antal ladhi laa yut’aaqu intiqaamah.
“O forceful, O powerful. O One with a severe might You are the One whose retaliation none could bear.”
After this, you blow in the direction of your enemy.