Gaia not only has invincible covering for single/random target overdrives, but also a powerful damage cut for the party-wide attacks. 40% for a party-wide damage cut is great! And awakening adds a free 20% thunder resist on top of that! That's 60% off against thunder in one skill.

SR Nergal does indeed have the self only invincible + cover combo in one skill. It ought to work against overdrives fine. What keeps her at SR level is that her defensive utility is just that (nothing for party-wide stuff) and the cooldown (12 turns is absurdly long). Still fantastic for an SR, but you do have to pay more attention to what exactly the enemy will do.