Terrain Defence

Terrain Defence is a gem effect in Xenoblade Chronicles. It reduces the amount of damage taken from damaging terrain, such as toxic swamps, lava, oil, and pure ether.

Overview
Dangerous terrain is not common, so Terrain Defence is situational. But in these situations, it can be quite useful, especially if there are enemies nearby that may be trying to bait (or force) the party into fighting too close for comfort. It also aids the exploration of areas rich in bad footing, lowering the risk of being caught by a battle before being fully healed, and making landscape traversal safer and faster in general.

Since terrain damage is based on percentage of max HP, putting Terrain Defence on a tank will reduce the raw amount of HP lost in case of fighting being expected. Conversely, AI party members cannot drop below 1 HP unless a battle is in progress, so reserving them for the player-controlled member is better for when enemies are not a factor.