You need the Eternal Support skill for this. To get it, go to Relewiese Hollow with a level 4 Sorcerer’s Ring. Search the area for the Star Rod, which has the skill. Make Estelle learn it. Be wary having the Anti-Magic skill on any character in the party will negate the effect. Go into battle and use Force Field until it takes effect. The easiest way to do this is to Overlimit spam it. Success grants invincibility for the entire battle. Thanks to Shinarouji for this!