Let's say I have a group of 10 units: 5 marines, 2 marauders, a tank, and 2 banshees. I want the marines to target the front line, the marauders to target some armored unit, the tank to target something else, etc. I know I can ctl-click to select all of the units in that group, but is there a key or some technique to tell each group of units to attack something different without losing control of the entire group or having them all attack the same thing?
You can do multiple control groups.
Group 1 = all of them
Group 2 = marines
Group 3 = marauders
Group 4 = tank
Group 5 = banshees
Selecting your other control groups does not lose your full control group. Very useful for air.
Set all your units to a single control group, for example 1, then cycle through the different sub-groups using TAB.
Another good tip is you can set all your buildings to a control group. Then when you are that the front line, select the control group once with the hotkey and cycle through the buildings again with TAB and get them to produce different units, all without having to return to your base.
There is no way to do this. In order to issue attack or move commands to part of a group, the others have to be removed from the current group.