Most of the shots seem to come from the saucer arrays, though they seem a little out of place from the Borg perspective of Locutus watching the battle.
One phaser beam from the Ambassador
class ship would seem to be coming from the secondary hull - which is consistent with how most Starfleet ships are designed, and how the Ambassador
was originally drawn, even though nobody ever got around to actually installing the ventral phaser strip in that model.
A purist might
argue that the source must be the deflector, as there are no other "features of note" in that part of the actual model. But I doubt that was the artistic intention here.