Unfamiliar with Feca

Fecas have 2 main shield spells that block damage: Rampart and Feca Shield. Rampart blocks a flat amount of damage on every hit (usually 100-200 dmg depending on level) Feca Shield blocks a percent of all damage taken, with the level 6 version blocking 35% of all damage taken.

If you hit a Feca who has Rampart, the spell will block a set amount of damage. For example purposes assume Rampart blocks 150 (the actual value may vary). If you use Elemental Drain on the Feca who has Rampart on and hit 300 with it, you will only hit 150 damage. Rampart will subtract 150 damage from what you actually hit (300-150 = 150). For this reason it's better to hit a shielded Feca with more expensive but more powerful moves than it is to hit them with multiple weaker moves, you need to pierce the shield.

The other Feca spell takes on the opposite role, protecting the Feca from higher damaging spells. If you use Elemental Drain again when the Feca has Feca Shield on and hit 300, it will do 35% less damage to the Feca, dealing 195 damage.

It should be noted that these spells do stack, and while I'm not too sure on how they stack, I believe that the flat reduction applies before percent reduction.

Damage numbers for Elemental Drain on Feca (assume Rampart is 150 flat reduction):
Without Rampart: 300 damage
Rampart: 150 damage
Feca Shield: 195 damage
Rampart & Feca Shield: 97 damage

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Fecas also have 3 conditional immunity spells:

The first of these spells is Truce. Truce makes all enemies and allies on the field immune for 1 turn. This spell is typically used to prevent something from happening on 1 turn, like Wrath turn, or to save an ally from their doom. Truce also has the major drawback of making the Feca go into the Pacifist state on the turn ending Truce. Under Pacifist state the Feca cannot do damage, but they can cast support spells.

Barricade is the Fecas close-combat immunity. For 1 turn the Feca cannot receive damage from spells or weapons cast from the 1 cell next to the Feca. If the Feca is damaged by any spells while under this effect from outside of the effective range, they gain 1mp (max 2)

Bastion is the opposite of Barricade. For 1 turn the Feca cannot receive damage from spells or weapons cast from anywhere but close combat. If the Feca is damage by any spells from targets standing right next to them, they gain 150% damage (max 300).

Barricade and Bastion cannot be cast together, so the Feca cannot be completely immune outside of Truce. Barricade and Bastion can both be cast on allies.

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The traps you see from the Feca are what we call glyphs. They work as a field of power for the Feca. These glyphs activate a certain amount of damage when you start your turn on them (Glyph of Repulsion is an exception and activates whenever you end on it).

All of these glyphs also have a side-effect with them. For example, Burning Glyph (the red square glyph) reduces the mp of anyone inside of it. These additional effects are removed upon exiting the Glyphs area of influence. Glyphs typically last 3 turns, so you should have ample time to escape from them, if the Feca doesn't lock you on them.

Link to the Fecas Wiki page

If I'm not wrong, Truce doesn't make everyone immune, it just reduces damage by ~5500, which means erosion dealers can still pierce through Truce.