User blog comment:God of nothing/Titan shifters-Invincible?/@comment-13234254-20131002135342/@comment-7418318-20131002145449

Even without transforming they can still regenerate, but as we can see with Eren and Ymir, the regeneration speed in their Human shapes is much slower than in the Titan shape.

So instant death wounds (decapitation, stab through the heart, ect) are most likely fatal to them and even wounds that aren't immediately fatal can kill them, if they die from them before they can regenerate.

And if I'd have to venture a guess I'd say drowning, suffocating, dehydration and malnutrition will work on them just like on any normal human, as long as they stay untransformed at least.