Mortar is a good option, it's simply a question of how many shells can you move from the loot pile to the mortar operator. It is genuinely a slow process, so starting the bombardment with one or two chem shells can speed things along.
EMP grenades, minibombs and even the superconductive cannonball can slowly and safely do the job from behind cover.
Direct fire weapons can be suicide; the turret will always begin the turn with a reaction score of 35, which is enough to react on any of your soldiers who fire sniper rifles or heavy weapons. Attacking at night will prevent it from reacting if you're hiding in darkness, then it's just a question of finding ninjas in the dark.
Never end your turn out in the open as the turret has sniper AI, regardless if it's a night mission or not. On the enemy turn it can absolutely attack anyone not hiding behind solid cover and LoF blocking terrain.
