In video games, it's quite common for a boss to only be vulnerable at a particular time. It's one thing if it's due to the boss needing to catch their breath every so often or the like. But sometimes, it's just because a given attack leaves them open. Maybe it exposes a Weak Spot while preparing the giant laser, maybe their attacks double as weapons or platforms that the player can use to reach their target, maybe on occasion it decides to just stand there laughing at you with its defenses down, or maybe it will keep charging head on into walls until it dies.