This isn't really a walkthrough, It is just a few hints that will hopefully help a little. I will try to get a Walkthrough soon.
Sell all the Colt .45s that all your unassigned officers have. This will give you about an extra $60,000 or so; this will help you gear up your units and pay for their training.
Each Assault Element can consist of up to five units: an Element Leader (required), a Scout (optional), a Rear Guard (optional), an Assaulter 1 (optional) and an Assaulter 2 (optional). Although it's possible to create an Assault Element that consists of a lone Element Leader, make sure you have at least a Scout and a Rear Guard.
Each Sniper Element can consist of up to two units: a Sniper (required) and an Observer (optional). There is no real difference between a Sniper and an Observer. They may be deployed separately or as a team. If you team them up, they won't shoot with any greater speed or accuracy, but in some circumstances they will lie with their bodies at an angle to each other, getting a wider field of fire.
Use Stealth Entry as your default way to enter doors. You should only use Dynamic Entry methods such as explosives, battering rams, or tanks as a last resort, or when you know that the lives of hostages are in imminent danger.
If an officer’s score is 100% for any given weapon, that officer will be 100% effective with that weapon 100% of the time. Take advantage of this!
It's a good idea to buy cheaper items like mirrors, Hooligan tools, and anything else you can fit in your budget for every officer. Wounded units can’t drop their gear, nor can gear be taken from dead or wounded units, so if your Scout is wounded and he’s the only one with a mirror, you’re in a world of hurt.
Tear gas grenades and flashbangs tend to bounce off vertical surfaces. If you line up your shot from the bottom or top of the screen, you stand a better chance of making the shot on the first try than if you try lining up from the left or right side of the screen.