After doing some investigation, it turns out that the Photon Drive units that contain Spirita Alpha, are the e-trial reward, and not an actual drop. Furthermore, the e-trial also provides pink arks badges, which can be used to trade for photon drive units as well, the more badges you trade in, the more slots it has. The 6 slot units that were mentioned earlier are a result of trading in 10 pink badges.
Haven't heard of anyone actually seeing the unit drop directly from a boss that was spawned from a trigger though, but if this is true, it would make sense that triggers that spawn more than 1 boss would be ideal, though these currently aren't available.