The following page of the FF7 Remake guide contains a description of the boss battle - Rufus and Darkstar. The fight against this boss takes place in Chapter 17 - Deliverance from Chaos.
Darkstar has 13,056 HP, while Rufus has 14,632. You are alone in this fight - be sure to have a Healing Materia and try to keep Regen on Cloud throughout the entire battle.
You fight Darkstar first. Here the best technique is Punisher Mode + Blocks. When his bond with Rufus is severed, he will get staggered pretty fast. At some point, Rufus will also start helping the beast by shooting at you. You have to avoid attacks and keep fighting Darkstar - it is best to stay within half a distance.
Time to fight Rufus. The opponent uses a firearm and cannot be injured in conventional combat. This can only be done if the enemy uses Reload and charges his attacks.
It is best to stay close to the enemy, run around him and avoid attacks. When the enemy uses Guns Akimbo, block, get close to him, and attack during Reload. In the meantime, cast Regen on yourself, and you will quickly deal with the boss.
Reward: 1,300 EXP, 10 AP, 1,300 Gil,