PoE2 bosses use telegraphed attacks, phases and arena hazards that test both your build and your mechanical execution.
Bosses have very high ailment thresholds, so freezing or shocking them reliably requires large hits or heavy investment.
Endgame culminates in pinnacle bosses: the original Arbiter of Ash and the new Arbiter of Divinity added in 0.5.0, plus each league mechanic's own pinnacle boss.
As of 0.5.5, Arbiter of Ash has Fire Resistance and Cold Vulnerability, and Arbiter of Divinity has Lightning Resistance and Fire Vulnerability, instead of All Elemental Resistances.
Cap your resistances and bring a healthy effective health pool plus reliable recovery before attempting pinnacle content.
Learn telegraphs and phase transitions; many deaths come from unexpected abilities rather than raw damage.
Consider a boss-focused skill and flask setup, since mapping configurations are often suboptimal for single-target fights.