I successfully used Berghia to eradicate aiptasia, but didn't have a hydroid problem, so I can't help there.
I will suggest, however, that if you get berghia, get at least a few and breed them in a 10 gallon tank. It is simple to do, but will take a few weeks for them to reproduce.
Also, be aware that certain fish (especially wrasses) will eat the berghia.
Are there many rocks that are infested? Can you remove the infested rocks and clean them?