Boss fights comes every three stages.

Basic attacks won't help you much in boss fights. Use one of Chuck's special skills: Thunder maneuver, Shadow and/or Push-Up.

Watch those cool down times for your special attacks and plan your moves according to it.

Runes you get during the game are used for your skills enhancement.

Upgrade your gear regularly especially before boss battles. Also, you can sell unneeded gear.

For more coins, games common currency, you can watch ad video.

Gems, premium game currency, are acquired by finishing games achievements.

Team up Chuck with a pet to fight with him if you can. Keep in mind it costs $4.99 real US dollars.