Since turrets can't walk you should be able to wear them down without taking any damage.
You'll be completely safe if you always end your turn out of their line of sight.
Just shoot at them, hide behind cover. End turn.
Pop out of cover. Shoot at them. Hide behind cover. End turn.
Repeat.
If your character has no ranged skills of any kind (Guns/Crossbows/PSI), then perhaps try chucking HE, plasma or EMP grenades instead.