Barriers are insanely strong if pushed to the max.
When a barrier enchant says 15%, that means 15% of your max health. So if your max health is 100 because you have no stats in it, you would get a 15 health point barrier. But if it were 300, you would get a 45 point barrier. The more health you have, the bigger barriers you will create. So use beds and other items to increase your health where possible. Using the scarlet ring to swap max focus with max health is a way to have 900 health, meaning a 135 health point barrier at just 15%. Note that max health decrease plagued enchants do not impact barriers since they do not affect the max, just how much of the max you can fill. Also consider damage mitigation. The more mitigation you have, the less the barrier gets consumed. So having 50% mitigation at 100 health is the same as having 25% mitigation at 200 health when it comes to how long your barriers will last.
Barriers from different sources stack. Barrier out of combat, barrier on kill, on damage dodged, on consuming food, and on parry all stack together. Use multiple sources to create the biggest barrier possible. Damage is split across all your barrier sources, so the order you create them does not matter. For boss fights, I find on parry and damage dodged to be most important and a way to keep your barrier up throughout the fight. For trash, barrier on kill is king. Barrier out of combat can be buggy, but is a good option for a plagued item where you don’t want to enhance the negative enchant. It can be used before a fight to create an initial barrier by simply switching main or off hand, at least until they fix swapping removing/reapplying buffs.
Barrier on food consumed is a good way to prep for a fight or create a barrier in a pinch, and can essentially replace better food sources. Consider having 300 health and 10% barrier on food consumed. You eat anything, even something like the starter food which heals for 70 health, and you get a 90 health point barrier. Once you improve barriers with gems and barrier gain plagued chest enchant, you create massive barriers with the trashiest food.
Barrier gain on plagued chest is HUGE. Having a 40% increase and beyond with gems puts barriers on a whole other level. With barrier on kill, it becomes really hard to die outside boss fights. Because the other armor pieces don’t have any barrier enchants when plagued, helm will probably be the only other item you might plague since barrier on consuming food for the helm is probably the least useful. On trash you will never need it, and on bosses it is rare to need to eat unless you never dodge or parry damage.
Lastly, because barriers are like extra health, they still work with thorns and damage on damage received. You are still taking damage, even if your health bar isn’t going down.