I use groups in conjuction with the stock formations, for example. I usually group all my cavalry but one or two units in one group to be my hammer-cum-panzer wing. This is especially useful for city battles when I just charge that one group through the gates and once enough of them get through they just sweep through anything that comes at them clear to the square. This is also useful during the field battles when it is the charge from these units that will sweep the enemy from the field at the end (or at the beginning, depending on circumstance).

As for infantry, my style varies according to demand. Ranged troops are definitely grouped on their own according to the missiles they fire. For minimal confusion I tend to create armies with only one or at most two kinds of missile troops to prevent fratricide. Shock infantry wise, if I play with a phalanx then I simply group the phalanx as one group. If I am phalanx-centric then I tend to make three groups, the central group to take the brunt of defensive and offensive attack, and two groups, one to either side, to flank. This enables me to subdivide centre, left and right for easy management.

If melee infantry are used, I tend to group my units in pairs or threes. On the macro scale these units can function in tandem as flanker groups-centre holding line etc, and on the micro scale they can work as two-unit tag teams, one unit menacing another unit's front while the other charges round the back, making the enemy turn around before the second unit hits the same unit. This is made even more devastating by the fact that both the AI and me have completely no concept of a coherent battle line, but tend to play Deep Battle style.

My groupings into left, middle and right do take into account unit type, through. I like homogenuous groups-if it's phalanx it's all phalanx. If it's melee it's all melee. And so on.