Being a Skaven Leader, I have very little in the way of tactics, but those that I do have, I will share here.
As I now have 3 Infiltrators, I make full use of them. Darc, the Night Runner with Sling, infiltrates to a great vantage point so that he can use his sling as effectively as possible.
Quirik and Skrat, my Assassin and Spear armed Black Skaven, form an early attack force or decoy force. They infiltrate to cover in a position that will either allow them to attack and ambush an enemy, or else threaten an enemy model. Either way is effective, as the ambush often leads to the death/K.O. of the enemy, while the threat of the assassin and Black Skaven is often enough to force models to either detour away or close on them. If they detour, they are often in a useless position for any ensuing combat. If the enemy gang up on the threat, then my whole gang can easily focus everything it has on all of the enemy gang.
My Sourcerer, Pzaap, usually has a bodyguard of swords-rats. This ensures that he can stay alive long enough to cast a spell.
The Slingers have the simple job of finding a place where they can bombard the enemy with stones as much as possible. This means that they often end up being deployed on upper levels, as they cannot then be easily attacked.
My final Black Skaven tends to look after the Rat Ogre to make sure that it doesn't suffer from Stupidity, while the rest of the swords-rats are joined by Ligit.
After the deployment, the battle is largely predetermined.
If the enemy go to attack Quirik and Skrat, then they will retreat slowly until backup and covering fire is available. This is an effective way of ensuring that your assassin can kill something while making sure that he doesn't get taken out.
If the enemy ignores Quirik and Skrat, then they have 2 choices. Firstly they can take out the enemy one by one as they move around them. Secondly they cause the enemy to disperse, and thereby cause the enemy gang members to loose their support.
The Slingers have the simple job of moving fowards into range of the enemy, and pelting them with stones, while staying out of harm by hiding and keeoing their distance.
Skritt and Raar have very few choices in a battle. Raar is bound to be targeted by all of the enemy's missile weapons, and therefore they advance at full speed to try to get into combat. If a Rat Ogre is not in combat, it might as well be sticking its nose in a mousetrap.
Pzaap and his bodyguard advance towards the enemy, loosing off spells. When they begin to get close to the enemy, Pzaap detatches himself from the bodyguard and allows them to advance alone. Pzaap only ever enters combat if he manages to cast Black Fury. Otherwise, he relies on his bodyguard to do the killing, as he is too weak and valuable.
The final group of swords-rats advance towards the enemy, helping whichever of the other groups needs backup. Skaven have the advantage of numbers. If you can pile enough Skaven into combat, you are almost guranteed to take an enemy Out Of Action. One of your Early henchmen can take the enemy down or stun him, while the later ones hit automatically and do the finishing blow.
I hope that was of some use, and remember, "He who fights and runs away, gets to squeak another day", or alternatively just poison the enemy leader.
Hive World Terra