Carrying capacity is a constraint
Consumables take slots, and slots are limited. That makes each one a decision about what problem you expect to meet, not a free extra.
Build your loadout around the expedition you are actually running, not a generic list of best items.
Spread the answers across the trio
Three players carrying three different answers handle more situations than three carrying the same thing. Agree who covers healing, who covers offence, and who covers utility before you leave.
A single problem the whole team prepared for is rarely the one that kills you.
Use them early in a bad fight
The most common consumable mistake is holding on too long. Items used after the first serious mistake are usually too late, because the situation has already compounded.
If a fight is going badly, spend immediately. Downward spirals rarely recover on their own.
Match consumption to pacing
Some expeditions are clean and need almost nothing. Others are messy and will drain everything you brought. Judge which you are in early, and do not waste stock on a fight you were winning comfortably.
Saving for the Nightlord only pays if you survive long enough to reach it.
Restock before the final fight
Whatever you spent on the way arrives deducted from what you have at the Nightlord. Check your stock before committing, not after you have already started the encounter.
Carry cures for the likely status
Fire and frost Nightlords punish teams that arrive with no cure. Pack at least one cure for each status you expect, because a DoT undoes your healing the moment you drink.
Spend buffs on the boss
Weapon buffs and damage boosters are wasted on trash mobs. Hold them for the Nightlord, where the multiplier actually decides the fight.