Tips for winning siege as Skaven?

Playing as Ikit Claw, I'm struggling a bit with attacking well defended major and minor settlement (full stack + garrison). My army is a pretty standard weapon team, ratling, jazzails, mortars, catapults.

In WH2, it was pretty easy. I just use mortars to kill units on the wall, finish the rest with ratling guns. In WH3, mortars seem to be less effective, AI units spread out a lot more and they also move to dodge my mortar.

More importantly, respawning towers, which I find to be really annoying, means I can't just take my time and try to shoot with ratling guns.

Should I bring a second army with melee units specific for tough sieges maybe? Just wondering if there are easier tactics to win sieges as skaven