36 skinks double tapping and hitting on 6's kill the HPA, statstically, in 1 round of shooting.
thats the key, now.. you can try to get 36 skinks in range in one turn or work with less and keep shooting at it. i have 20 skirmishers in all my lists, and usually 5-6 chameleons. you want to keep them near the HPA and dance arround it, if you are lucky he turns and charges them wich means you lose a unit of skinks and he loses 1-2 turns with his HPA depending on the distance it is from the main battle line.
i wouldn't suggest any other way. lore of fire is great against it, but no so good agaisnt hte rest of the skaven, and if you lose life, you buff the units in combat with felsh to stone anyway, so thats not exactly a tactic against it, its the usual tactic, and i should work. both flesh to stone and shield of thorns.
if you do go for fire, cast flaming sword on skinks and ignore its regen, and istead of 72 shots you'll only needd 36, thats arround 18 skinks.