Bosses and large enemies require patience
As much as it’s fun to go in guns blazing and aggressively attack bosses, consider being more calculated if you’re having trouble. The common theme in FFXVI is that all enemies have a limited array of attack patterns. Lock on to them and pay attention to their cues. Keep your distance and work on dodging these attacks on first encounter. Once you get a feel for them, go on the full offensive.