Magic
Proper use of the magical spells available to Kratos is essential for defeating bosses. Don't waste your magic against normal monsters, so that when you encounter a boss you will have a full magic bar. It is likely that a defeating a boss efficiently will require using a spell or weapon that you recently acquired in the game.
It is also imperative that you save the Rage of the Titans power for boss fights. Activating it gives Kratos an enormous increase in power and unlimited magic casting ability for a limited amount of time. It is a good idea to save the power as long as possible when fighting a boss because most boss fights consists of several stages. Using the power on an early stage can make later stages even more difficult.
Weapons
Your primary weapon for every enemy and boss battle should be the Blades of Athena, the default weapon. The Blades of Athena have good range, which can allow you to pepper bosses with hits without getting too close. Use the game menu to study the different moves and combos available for each weapon so that you can use them to the greatest effect. One of the best combos for the Blades of Athena is the Plume of Prometheus, which is performed by pressing square, square, triangle. The combo will do two fast hits and end with a long-range whip that explodes and deals fairly heavy damage and can stun enemies. This is especially useful for bosses that summon weaker enemies to fight you, like Theseus.
Weaknesses
Like most video game bosses, "God of War II" bosses often have a glaring weakness that you must exploit. At the beginning of every boss fight, your goal should be to find out your enemy's weakness and use it to damage him as much as possible until he moves into a different phase.
During different stages, a boss might have different weaknesses, so be sure to constantly asses what attacks and spells are working. If nothing else, it is a good idea to avoid the boss and observe its attack patterns so you know what to expect and when the best time is to try to deal damage.
It is also a good idea to learn which attacks you can block and which you cannot. It is often best to attempt an attack right after blocking a boss, since he will likely be close by and in between attack actions.