Eiro made sure to keep the holy flames centered around the area where 'the Death' would step out of that huge metal cube. And in the meantime, Koperia was relaxing and instead had her undead do all the work.
To an extent, Eiro expected that this was going to happen anyway, although Koperia herself was already plenty strong, the fact that she was going to use her own undead to fight 'the Death' was something that Eiro figured she would try to do. After all, if just a few of her undead were stronger than a true incarnation of death, then what would that make her? It would make her something that was above death. So, to gain that position, Koperia was going to simply hold back for all of this.
