I'll pile on a bit here. When I'm running the imperial side, I tend to stick to these as general rules:

• Always try to go after the FIRST rebel to activate with everything you have. Then focus on that guy till he's flipped.
  • That figure will not have the option to rest directly after taking a beating (because it's already activated), and it will be fairly difficult for that player to use that figure practically if you focus them down (they'll have to spend a lot of time resting them just to keep them upright).
• Use stun, bleed and weaken as often as you can to remove actions and slow the rebels down.
• If you can, pile swarms of figures into hallways in front of objective doors or on top of objective tokens/terminals. This will force the rebels to chew completely through you in order to interact with the objective.
• Don't always feel like you have to spend every point of threat. Be strategic about deployments - if you can wait one round, even if the board is clear, to deploy 2 full groups of troopers, it's better than deploying a single officer who will immediately die and only getting 1 of those trooper sets out instead...

Those first 2 are the most important (for me). An IP who focuses one one hero at a time is hard to beat...