I've been struggling with this as well, having just started playing as IR after playing every scenario (except FTGU) as the NR. The IR definitely requires a more complex approach than "spam freerunners and Speeders/V-Wings, add 1 Jedi" for someone interested in getting the ground battles over with as quickly as possible.
So far, I've mostly been using a variant of Greystar's first force. I've only played in Era 1 so far.
1 Stormtrooper
2 AT-ATs for anti-vehicle and disposable scout stormtroopers
4-6 AT-PTs for anti-infantry and anti-air
1-3 IDTs depending on map/opposing force for airpower and scouting - the AI as the PA seems particularly allergic to building any useful anti-air units, so I usually set up a chokepoint of AT-ATs with AT-PT support and use 3 IDT groups to wreck their base.
0-1 Specialists - for repairing AT-ATs and AT-PTs on maps without handy build pads for repair stations
0-2 2-M Repulsor Tanks for scouting where disposable stormtroopers die too quickly and there's too much anti-air to get an IDT through - used mostly against the NR, also handy against opposing air units when massed