General Army Notes:
Your average human regiment in the Empire army has 3 Attacks, 3 Wounds, and 3 bases in a unit. This may not seem like much, but the Empire army's strength comes in its versatility.
Foot Units:
Halberdiers: Average stats. You must take at least two units of Halberdiers. Halberdiers form the core of an Empire army, and will actually stand up relatively well in an assault, even with their unimpressive stats.
Thunderers: Average stats, one shot per base. Their guns give a -1 modifier to enemy armour saves. Thunderers give you a decent missile unit that can support your troops, but they will not win a battle for you.
Crossbowmen: Average stats, one shot per base. Two units are mandatory for your army. Use them in a supporting role, lending supporting fire for your assault troops.
Flagellants: Flagellants always charge on initiative and cannot avoid an enemy unit on initiative. They do not fall back to missile fire and are not affected by Terror. You can only have one unit of Flagellants, but their assault value can help your Halberdiers and other units break what might otherwise be a stalemate assault.
Skirmishers: A base of Skirmishers is always attached to an infantry unit and they gain the armour save of that unit. Skirmishers can either lend more punch to an assault unit or aid a missile unit to survive assaults.
Cavalry Units:
Knights: Knights, being cavalry, have a fast move and are incredibly tough in an assault. However, Knights still only have basic stats, so they will not be decisive in an assault alone. Knights should always be supported.
Pistoliers: Pistoliers are excellent supporting units, with the ability to fire and assault both, and can move fairly fast. Pistoliers supporting units of Knights can be a deadly combination! You can have up to four units of Pistoliers.
War Machines:
Hellfire: The Hellfire is most useful when doing close-range support (such as supporting fire for other war machines).
Cannons: The cannon ball can bounce behind the hit unit and might end up damaging, pushing back or confusing more enemy units. Cannons can give some decent supporting fire for your assaults. It should also be worth noting that if you stand and shoot the cannon does not nullify enemy saves and does not bounce. Give it a supporting unit to protect it from assaults.
Steam Tank: The Steam Tank is fairly effective as a mobile cannon, and can stick around with its 3+ save. Its speed (20cm) allows it to move short distances in a relatively short amount of time, meaning it can get to where it's needed when it'si needed.
Characters:
General: The Empire General's ability to aid units in combat is second to his more important role of giving orders. Don't get stuck in with the General and any unit he is with unless you are sure you can win the combat.
Hero: You can take two Heroes for every 1000 points. The Hero is great at lending his weight in an assault and giving orders.
Wizard: The Wizard's ability to cast Battle Magic spells is his main strength. Protect him!
Griffon: The Griffon's main usefulness is in giving a characted riding it the ability to move around the battlefield quickly, but its relatively high cost can be a drawback.
War Altar: The War Altar allows a wizard to roll two dice to cast one spell per game. It can be useful for casting a spell when you really need it.
Special thanks to Dalkwave for his help on this tactical overview!