Not all weapons two hand. L1 transforms your weapon. Some two hand, some extend, some do other things.
When you one hand, your L2 is usually a gun. There are some other L2 items, like a torch, but mainly guns. Most are shit for damage without stat investment, and can’t be aimed outside of the lock-on feature. Blunderbuss can stun and interrupt enemy attacks, though. You parry by shooting the enemy right before an attack lands, similar timing to a Dark Souls Parry. High Risk, High Reward, also like Dark Souls. Main difference is ammunition is limited - 20 rounds, just like Blood Vials. Ammo can be bought or farmed. Tapping up on the D-Pad will trade health for a few rounds of ammo, if you are out.