Every hero has a unique tactic, though they fall under certain categories. Here are some tips for maximizing your hero's tactic based on the type of tactic:
Examples: Angron the Conqueror, Asketill
These heroes' tactics do direct damage to nearby units/buildings or give your hero extra damage output (and sometimes have other effects) for a short time. Ideally, deploy this early on to get the most benefit.
Examples: Phadrus, Arcadius, Ekkehard the Breaker
These heroes' tactics boost your units damage/speed for a certain period of time. Deploying these tactics at the right time is key. Ideally, deploy it just as the units are starting to fully engage the enemy base. The sooner you get the bonus, the more units you will have who benefit from it.
Examples: Saul Tarvitz, Babak-Tu
These heroes give your units damage reduction or dodge abilities for a time. Deploy them just as units enter turret range. If you can get the damage reduction earlier on, you can fight with your whole army longer.
Examples: Ekkar, Calas Typhon
These heroes' tactics heal your units for a time. Deploy them shortly after your units have started taking turret fire. You want to deploy it early enough that most/all of your units can benefit, but not so early that your units haven't yet taken significant damage. The exact timing depends a lot on the unit type. For example, with Glaives, wait until a few of them are in the red. For lighter units like tacticals, you may need to deploy it earlier.
Examples: Garviel Loken, Zere Mekonnen
These heroes resurrect some or all units that have died. Ideally, deploy when you are about most of the way to the end of the battle. That way you will resurrect as many units as possible, and also still be able to use those units.
Examples: Brother Gundor, Cassius Windshroud
These heroes deploy free units somewhere on the map. If possible, use them to draw fire away from your main army.
Examples: Nevian Doombringer, Semprius Snaketoungue
These heroes give you extra combat energy or reduce the energy cost of using tactics. Use them right as you're about to engage the base or a little later. For Doombringer, use as many tactics as you can in the time window.
Examples: Brother Zhyrgal, Veltin the Zealot, Tibor the Warhound
These heroes teleport across the map. This can be useful to take out a few turrets that are off the main path (e.g. beamers or whirlwinds). Be careful not to send your hero too far ahead, or all/many of the turrets will target him and you risk losing him. Also remember that when you have a talent that benefits units near the hero (e.g. Psychic Drain or Possessive General), you want your hero to stay close to your army.
These heroes are especially useful in skirmish as they can quickly get out of trouble, and get around blockades.
Examples: Horus Lupercal, Ezekyle Abaddon
These heroes teleport across the map and bring your whole army with them. Teleporting into the middle of a clustered base can be deadly (especially with Ezekyle). Make sure you've disabled sufficient turrets or jump closer to the edge of the base. Also beware of splash damage turrets as all your units will initially occupy the same space. If you drop right next to a flamer, for example, all your units will take full damage from the flamer immediately after teleporting. Also, make sure to clear the landing area of any mines (e.g. with a deathstorm), otherwise they can quickly kill a lot of your units.
These tactics work best with high-dps units since you will have a limited amount of time in which the turrets are cursed/haywired.
Archmagos Draykavac's tactic is unique. Since he tends to stay behind your army due to his long range, his tactic is not very useful in the later part of the battle. Often your army will clear turrets out before Dray gets in range. His tactic is best if you are going against a base with all the turrets near the drop zone. Deploy Dray and your army and immediately use his tactic. Since you are dropping him right at the edge of the base, he will dominate many or even all the turrets in the base.
Silvian the Swift's tactic is unique in that it draws the enemy hero out of the scriptorum, without having to destroy it or move a unit within range of it. It is especially valuable when dealing with a particularly deadly hero (e.g. Draykavac if you are using vehicles). Deploy Silvian and use his tactic right away to assassinate the hero. It's best to teleport a little in front of the base so that if Silvian fails to kill the enemy hero in the duel, your units can target the hero afterwards.
In skirmish, Silvian's tactic can be used to kill enemy heroes with impossible wargear. This can significantly weaken/slow down the enemy units. Again, use it right away to get the most benefit.
Tip: Some heroes' tactics do friendly-fire. See Appendix A for details. Use them with caution!