... Probaly the usualy asnswer to big scary stuf.... Kill it with lots of skink poison shots. Ps. (5 units of 10 blowpipe skinks posible 100 shots)
Kill it quick though. I think the best way is the poison shots. However, you could also bog it down with skink cohorts. Once it takes a few wounds, it becomes as much a problem for the skaven player as for you.
Skrox are the best unit to take it out if you don't have tons of shooting available. On average, 10 wounds are needed to drop it due to the 4+ armour save so that equates to ~100 shots (oops, I mean ~60 - bad math day)! However, every time it takes an unsaved wound from ranged fire, it has to take a control test and goes crazy on a 1 so you might get lucky. The DW is fairly easy to break in combat however with static res as it doesn't have thunderstomp and the S6 attacks from krox don't hurt either. The biggest problem is running it down since it flees a full 3d6 and can shoot again if it rallies.
skrox and magic missles. it is hard to get enough skinks nearby without losing them all. Random movement doesnt allow you to take a charge reaction so if you are close enough to shoot it, it is close enough to get a free charge on you. Random movement is probably the best way in the game of cutting down skirmishers, and poison is a great way to kill chariots and big monsters, this is why my skaven friend and I always hate playing each other. One in six chance of poison means it takes 60 shots to cause 10 wounds.
Skrox may not be the best idea since, its lightning can be used in combat, which can will then on average kill a kroxigor per skaven turn. While the kroxigor may be able to kill it, there is a good chance at least one of them will go with it.
I Metal it off with Searing Doom and then some poison. You can also just railroad the thing with terradons until it's dead.