Helstorm Rocket Battery

Helstorm Rocket Battery is a Empire siege engine unit in Total War: Warhammer. An erratic flurry of explosive airborne destruction - Helstorm rockets are extremely powerful, if extremely unreliable, weapons from the Empire's arsenal.

Description
The Helstorm Rocket Battery was inspired by a particularly impressive fireworks display. Early prototypes blew apart an entire floor of the Imperial Engineers School, but the soot-blackened Engineers persevered and succeeded in creating a deadly, if unpredictable, weapon. The shrieking rockets fired by this bizarre machine are wildly inaccurate weapons that have little chance of hitting anything other than (eventually) the ground. However, when these rockets do land on target, the results are devastating; entire regiments are blown apart by a series of earth-shaking explosions. After Elector Count Boris Todbringer was almost killed by an errant barrage, they were dubbed 'Helstorm' rockets after the colourful language used to reprimand the unfortunate crewmen.

Strategy
Helstorms are artillery meant for two things: harassing the enemy as they come to you and harassing rallying units returning to the field. This weapon has a very long range which you should use to its maximum. However, it is also very inaccurate, frequently missing targets by a large margin. It is designed to saturate an area with explosives rather than target a specific unit.

If using this unit, do not advance unless you must. Target this artillery in roughly the center of the enemy's most densely concentrated units (normally the center of their line, with their lord, missiles, and heavy infantry), especially where the greatest concentration of non-monstrous creatures is. Let fly! It is also very armor-piercing, meaning that you should target armored units. However, do NOT target enemy units within about 25 yards of your units; it is just as likely to hit them. Once the enemy begins to rout, instead of sending your cavalry out after them, you can have the Helstorms target them, saving your cavalry for the valuable hammer-and-anvil charges that the Empire thrives on.

Alternatively, send out a unit of Flagellants at the beginning of the battle once the enemy is about 200 yards away. The AI (and unskilled players) will frequently send at least a couple units of heavy infantry after them, forming an excellent target for Helstorms to shoot at.

Useful but needs to be babysat
This artillery does a lot of work against advancing enemies, but once you send skirmishers out or they get within distance you need to take command of the unit to keep it firing at a far away target, like missile troops. Otherwise it could start firing into your front line, always causing far more casualties to you than the enemy.

If the enemy has absolutely no troops that aren't in melee, either order a stop fire or target the ground (Alt-right click) about 10 yards behind your enemy's last soldier. This should help to compensate for the poor accuracy and make sure most of the shots either hit your enemy or go harmlessly past.

Heroes and Artillery
With the heroes of the Empire as shields, there is a way to use a rocket battery to pour all the firepower into the enemy without unnecessary sacrifices. This method is very powerful, but the level of heroes greatly changes the combat power of roster.