Best place to do this is in the leveling zones for your alliance, zones 2 through 5. See the following for where to find the generals:

http://tamrieljournal.com/eso-general-executioner-daedric-red-dye-guide/

Once in Caldwells Silver and Gold, all the generals will have a chance to show up and that makes finding a specific one harder.

And, please, if your over leveled for the dolmen and other on level players are around, let them have most of the combat. Only step in if you notice them needing help. You only need to kill one mob to get Dolmen credit, and you only need to get one hit on the general for that credit. Although most generals are such wailing on them with everyone else is a good thing.