Bretonnia relies on its cheap, but moderately effective infantry to hold down enemy units so its very powerful and numerous heavy cavalry can deliver killing blows. They use very old-school medieval tactics - think Medieval France or England (before the introduction of the welsh longbow.)

The Empire is much more based around infantry, and the selection available to the Empire is far more flexible. Swordsmen are well trained and can easily hold their own against elven units, where halberdiers are able to crush enemy armor and kill orcs more effectively. The Empire also has handgunners, which are devestating ranged units, great for killing heavily armored expensive shock troops. They have a vast selection of very effective artillery, and their Knights are heavily armored and some of the better heavy cavalry available to any faction. The Empire also has its oddball units, Greatswords which are heavily armored shock troops that almost never run, and Flagellants, Crazy Sigmarites which never never run. They've even got mounted skirmishes - Pistoliers and Outriders! Yes, mounted and armoured nobles armed to the teeth in all manner of experimental and sometimes reliable blackpowder weaponry.

The Empire can really use any tactic based on the unit choices, they have such a balanced and flexible army, which is partly the cause of their popularity. The only problem with the Empire's flexibility is the lack of super-soldiers and monsters that many other factions feature. (trolls, ogres, daemons, swordmasters, bone-giants, etc...)

This is assuming we omit the Steam Tank