What? One battalion of champions would destroy several battalions of CS.
Which is why battalions would need to be a togglable feature, similar to grouping in AoE1. Just in a proper formation.
You'd select a group of units that is eligible to form a battalion and you'd click on a "Form Battalion" button (or use a hotkey). Now, any time you select one of the units in a battalion, you will select an entire battalion.
Similarly, with a battalion selected, you would click on a "Disband Battalion" button to un-group the units. It's simple in theory, but the implementation will be more complex (as will be adding any bonuses to units in a battalion).