My best advice, if you matter in this guy’s life at all, thrive.
Be the best you.
Be happy.
Be motivated.
This will hurt worse than any curse.
And if you don’t matter, why should they?
Any curse is the best curse. What fuels that curse is what determines everything. If you’re weak willed your curse will be weak, if you’re furious and your will is law persay the curse will be one hell of a curse.
As said previously, how powerful a curse is really depends on the magician, not necessarily the curse itself. Some of the “simple” curses can be quite shattering to the target if all the rage can be summoned, channeled into building the manifestation, and, more importantly, let go afterwards so you may go on with life. You’ll drive yourself mad obsessively waiting for results otherwise.
As far as spirits that can be of help, one’s own ancestors are ones often overlooked. Although Andras would be a good one as well