The following aspects are changed in Hard mode. The best tactic in Hard mode is to complete the area without worrying about enemies. Finish off the enemies and do not worry about making high scoring kills unless there is only one enemy. Also, if you know where any extra lives are, pick them as soon as possible. If you managed the complete the game, pick up the special weapon awarded for defeat the Black Baron. You will need them to survive.

Enemies are far more relentless with attacks.
Enemies have now mastered the art of dodging and blocking.
Enemies now have weapons equipped to them (most of the time).
Enemy HP and attack power are increased.
You will only have one life per level.