If you're struggling in this game, focus on getting high combos without breaking the chain for a higher score.
USE THE INVINCIBILITY FRAMES OF THE SPECIAL ATTACKS TO AVOID DAMAGE.
I cant stress that second point enough, dont be afraid to use special attacks in sticky situations. If you land enough big combos throughout the level then you should have enough points for an extra life before the level is over.
If you can get through the level without losing a life, while earning an extra one in the process, then you should have enough to beat the stage boss easily.
Its really not hard to get through the game with no assists.