"Listen well, man-thing," snarls the rat lord. "My army is march-marching to war! I have many choices to make. This is how I pick-pick what forces will kill-kill my foes."
The most important part of a Warhammer army is your rank-and-file troopers. This is especially true with the Skaven as their Clanrats determine how many other types of units you can field.
Per the Mainstay rule, the number of Clanrat units cannot be outnumbered by any other type of unit. Additionally, your army must include at least one unit of Clanrats. However, even with only a single unit of Clanrats, you can still field a wide variety of troops.
The optimum size for a Clanrat unit is between 25-30. I prefer to field them in units of 30, five models across and six ranks deep. This means I have a wide fighting frontage and I can loose 11 models without having my unitís Leadership affected. A banner and musician is almost mandatory and a Clawleader will add an extra attack to the front rank.
When arming your Clanrats there are two schools of thought. On one hand you can give them spears so that the second rank can attack when defending or after the first round of combat. However, you must decide if the extra cost is worth it. Without the spears, the unit has a 4+ armor save in hand to hand and is one point per model cheaper. Personally, I opt for the spears for the tactical flexibility it allows. I can choose to fight a combat with either spears or the hand weapon and shield combo, whichever is more viable.
Now you must decide on how many units of Clanrats you will field. If you plan on swarming your opponent with a horde army, your force should consist of as many Clanrats as you can afford with an equal amount of Clanrat Slaves. I prefer to field a large army, but more diversified than just Clanrats and Slaves. I usually field three blocks of Clanrats.
Weapon Teams can be attached to Clanrats and Stormvermin units. The Ratling Gun is by far the most popular choice. You donít roll to hit, you have total control over how reckless you want to be with it, and if a Misfire is generated it has a very forgiving Misfire Chart. The Warpfire Thrower is more expensive and has a one-in-six chance of Misfiring. A Misfire practically guarantees youíll loose the Warpfire Thrower so fire only when you have a good shot. Keep in mind the template is 8Ē long and an average roll will shoot it out four to six inches. Additionally, any unit taking casualties from it will need to make a Panic test. It is very useful against large units where you can hit a large amount of models. The downside to each Weapon Team is that they are Move or Shoot units.
Clanrat Slaves have many excellent uses. Primarily, they are cheap! They can serve a variety of purposes from shielding your other units from missile fire to baiting out Frenzied units. They are also good for tying up an enemy unit so that you can shoot it with Weapon Teams or Warp Lightning spells or flank charge it with another unit of Skaven.
Personally, I donít give my Clanrat Slaves any upgrades except for a Musician. Invest in more Slaves instead of upgrades. I find fielding units of Slaves with the minimum number of models the best. Why waste the points? Face it, they are going to die anyway. The only good thing about it is that they wonít cause Panic tests when they do.
Stormvermin are the elite of the Skaven army. As such you can only field a single unit of them. Sadly, their stats arenít all that elite! They are more expensive than your other Core choices. I usually field a unit to act as my Warlordís bodyguard. Stormvermin units perform well when fielded in units of 20-25 models. My unit is five across and five ranks deep.
With their halberds they are Strength 4. Also, they are your only unit equipped with heavy armor. You can give them shields, further protecting them from missile fire. It also allows you the same tactical flexibility as with Clanrats. You can opt for higher Strength attacks or better armor saves (3+).
Stormvermin are one of two of your units that can carry a Magical Banner. My favorite choices for them are the War Banner or Umbranner. With the Umbranner, you can shrug off a lot of the missile fire this unit attracts. Even against an opponent who doesnít utilize a lot of missile troops it is useful, as it protects the unit from wild Ratling Gun fire.
Rat Swarms and the upgraded Plague Rats are good for tying up expensive enemy units, as they are Unbreakable . The Plague Ratsí Poisoned Attacks mean they can harm high Toughness opponents. The Swarms are Skirmishers so they are great for protecting flanks as well. Since you can only field one unit of them, I prefer to field them in units of three to five bases. It all depends if I want them to protect my flanks (the smaller unit) or tying up a really tough enemy unit (a full sized unit).
Poison Wind Globadiers are excellent choices against well armored and/or tough opponents since their globes auto wound on a 4+ with no armor saves. As a Skirmishing unit they have a 360 degree line of sight. Granted, their poison wind globes only have a range of eight inches. I usually field my unit with eight models. For a little added punch, you can put a Warlock Engineer in the unit armed with Death Globes.
Giant Rat packs can be fielded for different purposes. A large unit of them can be used to tie up enemy units like Slaves. The downside is that Giant Rats will cause Panic tests if they break, flee, etc. Smaller packs can be used to plug gaps in your battle line and for flanking attacks. When using them for flanking I prefer to field them in units of two packs. That way I can loose a few models and still be able to negate rank bonuses and cause Panic tests.
Night Runners are another versatile unit. They are fast and have a 360 degree line of sight so they are good at protecting your flanks. If you donít give them any upgrades, they can make a cheap and effective missile screen. However, they have a number of weapon upgrades to allow them to fill different roles on the battlefield. An additional hand weapon gives them more punch when they do attack the enemy. Slings allow them to fire with better accuracy than Clanrat Slaves. Throwing Stars, while short ranged, can be effective in large numbers.
I prefer to field Night Runners as a combination screen and war machine hunting unit. They are fast so they tend to get shot at first. As Skirmishers the effectiveness of shooting at them is lessened. With 15 of them armed with additional hand weapons, I am usually guaranteed that a large number of them will reach my desired target. Keep in mind, that as a Skirmishing unit they will not negate rank bonuses when they charge an enemy unit in the flank, though they are useful in adding numbers to your side during a combat.