As somebody who plays a lot of skaven, I'm here to tell you: rats have more guns and all their shooting will out-range yours. HOWEVER, skinks are extremely useful for their shooting ability, and they tend to be the best way to deal with the big stuff (e.g. abomination, doomwheel, etc.). Magically speaking, a Slann will out magic a grey seer any day, although I dare-say either of the skaven lores is overall better than lore of heavens. Not saying heavens is bad, it's just more prone to rolling spells that don't help much (but then, I'm biased). The only thing that should be charging the furnace in the front is an oldblood with either blade of realities, or a carnosaur. Otherwise, charge those disease-ridden vermin in the flank (or just tow them about the field with frenzy). Skaven ranked units in general fall apart very quickly to flank charges, the exceptions being stormvermin (since they have REAL armour) and any units pushing a bell or furnace.
A couple general notes:
Charging the bell in the front is actually not a bad idea if you're looking to kill the seer. He's relatively fragile and definitely worth it.
Your basic magic missile (i.e. 1d6 S4) is typically enough to take out a weapon team if you need one gone.
If you take a steg then don't forget the terror.
Also, don't take slaves too lightly, they STILL hit on 4's and are S3, so they can do some damage if they get the charge, and enough of them can give you grief even when you break them (yay, exploding...).
The unit I'd be the most concerned with is censer bearers. Those guys hit hard! Remember, he knows most of his units will lose a straight up fight against saurus unless he's really lucky, so be prepared for baiting tactics. Sometimes it's better to let him come to you.
For magic lores, I tend to favor lore of death, mostly because of Drain Life, although walking death on your skinks might be effective (and I just find it hilarious). If he's running a gun line, lore of life is also nice (for stuff on hills), and lore of beast is great against the abomb.
i jsut meant that a carnosaur challenged by a bringer-of-the-word (champion) will kill 1 model and even if he wins the monks will remain in combat due to unbreakable and the plague priest will strike 1st next turn (i think he has bigger I than plague monks who have 3).