Thanks for trying it out!
Yep, bosses have about half as much hp in Review Mode (single player mode) compared to normal. However, the health are balanced around players using the stronger guns.
Good point on the issue with other powerups. The shield/walls are meant for player to use to dodge boss's full map attacks. Maybe I should have further buff melee to deal even more damage to compensate for its lack of range. Thanks for the feedback