Ironskin Potion

Defense subtracts a fixed amount from every hit you take, which means the value swings wildly with what is hitting you. A swarm of weak enemies loses a large share of each hit. A boss that removes a third of your health per contact barely notices the same subtraction. Judge it by how often you are being hit, not by how dangerous the fight feels.

Endurance cuts a percentage instead, so the two trade places depending on hit size. Below a certain threshold this one is doing more work, above it endurance is, and the threshold moves with world difficulty. For any fight that actually matters, drink both. Neither is expensive enough to ration and they do not overlap.

You should never be short of these. The ingredients are common, pots and chests hand out spares constantly, and slimes occasionally cough one up as a bonus drop. Most players end up with a stack they never deliberately crafted.