The Nucleic Core boss has four types of guardians to impede your heroes;
The Templar (name under review) will directly guard the core and apply stuns to heroes to draw out the fight
The Spiker (name under review) generates damage reflection on itself and allies
The Flower (name under review) debuffs your party's random target chance and dodge
The Jaws (name under review) deals significant direct damage to heroes, and reacts violently when it sees that the core is struck.
As for the core itself, it will be providing the majority of stress your heroes receive, while it spreads the Leeching status to your heroes.