As a melee attacker, just make sure to move away as soon as you see her glowing to avoid getting caught in the blast. Once she has fallen down, you have a coupled of seconds to do damage against her. She will then recover her barrier afterwards. She will glow before blasting an explosion of light which deals a significant amount of damage.
This attack happens when Rennala is about to recover from having fallen over. Still, it’s best to be wary of these attacks as getting hit with enough of them can still be fatal. The good thing is that they don’t attack as much, and when they do hit, they do not hurt that bad. Rennala’s children can also attack by shooting flames or by throwing books at the player. Position yourself behind the bookcases so that you can use them to block the tombstones. Once they are being summoned she will be repeating a dialogue regarding her children being “born anew”, signaling that she’s preparing for this attack. After a few seconds, they will rain down on the player. While shielded, Rennala will turn some of her children into tombstones that float in the air. Once all of them are killed, Rennala will lose her barrier and fall to the ground, briefly becoming vulnerable for a few seconds. These children aren’t tough and can be killed with a single hit. Some of them can be hiding behind the bookcases, so make sure to check take a peek in those corners, too. These children are the ones with the golden aura surrounding them and there are usually three of them casting the spell. In this first phase, you have to find her children that are casting the barrier and kill them in order to make Rennala vulnerable. Rennala will be invulnerable at this point, shielded by a magic barrier and is floating high enough that no melee weapon can hit her. On the second phase, Rennala can summon several kinds of spirits that can aid her in the fight.
Her children can also attack using fire spells and books as projectiles, but they are only present in the first phase. Rennala uses ranged attacks in both phases of the fights.
Elden Ring: Rennala, Queen of the Full Moon – Attacks and Strategies