You can craft binoculars and equip them on a pawn with high shooting skill, they mark the target and remove indirect fire penalty of mortars and use the skill of the spotter instead of mortar operator for accuracy, this should give you much better odds of actually hitting where you want to.

in my experience, with a spotter i can land all rounds within like 4-7 tiles from where i marked, very often right on the spot as well

Oh, I had no idea. That's definitely something I need to try.

If you can outrange them with an anti-material rifle, use the HE ammo, it will wreck most turrets in 2 or 3 hits. Otherwise use the following tactic as a general rule:

1) Lure all mobile forces out and dispose of them to your liking.
2) Use smoke grenades in a Milkor for instance and direct your pawns to stand INSIDE the smoke screen.
3) Once again, load HE ammo for a couple pawns with machine guns. - the smoke will cancel turret target aquisition and you can shoot them (albeit with lower accuracy of course)
4) make sure than you refresh your smoke grenades, because usually you cannot mow down the large turrets in one smoke duration.

Cheers

PS: Mortars are too inaccurate and WAYYYY to resource intensive IMHO.

HE ammo works wonders against structures (walls, ship parts, mech clusters).
Look for ammo with (Bomb) damage on the description.

I just took to manufacturing heavy rocket launchers, LAW and/or RPG-7, always had a Doomsday or two on standby. If we were going to attack a mech cluster, it was methodically planned out before the first shot was fired. Everyone in position for a simultaneous rocket strike on all heavy turrets and centipedes, then drop the tubes and dash behind cover while the machine gunners and anti-materiel rifles open up on whatever's still standing. Pure alpha strike, not getting into a battle of attrition with robots.

Alternatively, jump packs and monoswords. One small cluster was cleared out by basically one soldier because she could get fairly close due to all the ruins in the area, and then jump jet into the turret's minimum range where it was helpless, and the other turret couldn't draw line of sight due to the first. Once it started sparking, she just jumped into the second turret's dead zone.

I've also heard that EMP mortars and grenades work well.

Agree with UnhOld: the simplest and earliest solution is smoke. Smoke grenades and smokepop belts make them unable to target. Just keep laying down more smoke while your pawns are still in it, and you can walk right up and melee them to scrap, assuming you've taken out the mobs (mobiles) first.

Note: in 1.2 with CE they can still fire at you if they were already targeting -before- you lay down a smoke screen, so be careful to start spamming from around a corner when they aren't busy aiming at you. Haven't tested this in 1.3 yet, but it's still a reasonable precaution to start smoking from a "safe" angle.

smoke nades and belts