So how do Lizardmen best deal with an Arachnarok Spider? I'm coming up against one in my next game (1500 points) and the best thing I have been able to come up with for it is an Oldblood w/ Blade of Realities on a Cold One or a large unit of Chameleon Skinks. Does it have any weakness other than low leadership that can be exploited?
No point trying to kill it with conventional attacks, it has 8 wounds and a 4+ save. I'd magic it. Slann w/Rumination and Mystery just fits into Lords. That gives you 'Miasma' and 'Pit'. Cast 'Miasma' first, to reduce it's Initiative down to 1 or 2. Then hit it with small blast 'Pit', even if you scatter bad you should still usually touch it with the template (it's a giant model). Then he has to roll a 1 (2 if you only reduced it to I2) to not die. Your alternative of an Old-Blood w/Blade of Realities is a good option, just remember to take Potion of Speed on him so he'll go first (it has I4).
I've had some epic battles with the a-rok. Temple Guard will tear it apart eventually, even with the 8 wounds, lore of life buffed TG just don't care. It just might take an extra round or 2. 3 krox in a unit of skinks also does a good number on it as it cannot get in its thunderstomp attacks so your 2-3 wounds (on average) per round from the Krox will always count.
I battled one at the North Star GT in November. My ancient Steg sent it packing. Didn't even have to buff him or anything. Of course, maybe I just got lucky.