There are only three AR's that are arguably worth taking the scope on and they are the Argus, Valkyrie and Revenant. The Saber should nearly always have mag, extended barrel, and AP or Drill rounds. You can rarely go wrong in the AR class with that mod and ammo setup, although some are so wimpy Drill isn't a great idea, and the first three are relatively inaccurate relative to their other stats and want some accuracy on most classes.
There aren't hardly any builds that are good against bosses but not against mooks. This is because DPS is king, and if you can drop a target with 48,000 HP efficiently you can probably do the same to one with 1700 or so.
The reason the reverse isn't necessarily true is because low tier units do not take much damage to kill, and a low to moderate setup in terms of dps can still potentially one-shot mooks, or shoot-power mooks, or whatever.
There are only a couple of units where it is actually prudent to aim for the head with the Javelin GI. One of which it usually makes sense to stop in her tracks, so it is easy. Granted, it is slower even if you don't miss compared to several other GI setups.