my suggestion is to make a mission that must be completed for every player that wants to go to do elites.
I'm not sure what such a mission would involve? Have some of the STF structures on a mission map that behave as they do in STFs? Example: 4 Nanite Generators around a Transformer and a nearby gate that spawns Nanite Spheres or something?
I'm not sure of the effectiveness of that. You can demonstrate to someone 10000 times that blowing up a Nanite Generator spawns Nanite Spheres which are bad, and it may still not cllick that the most efficient way around it is to use the 10% tactic (just for example). Someone has to tell them. Someone had to tell me.
And quite a lot of the tactical details differ from group to group anyway (maybe not on Infected so much, but there are definitely a few variations for Cure and KA). You couldn't realistically make a mission that'd teach them.
You can test their build quality to a degree, but that's about it. But with regards to build quality, a better solution would be to implement more of a gradually increasing PvE difficulty curve than what we have now: