Scratch

A big sprite in the middle is good of the stage is good as it draws the players attention to center stage. A change in music to a faster paced track also gives the impression of a boss fight (the pokemon games use this, for example).

Depending on the boss, phases are good. For example, at certain points during the battle the boss behaves differently:

above 50% health, damage = 1, rate of attack = 1 per second
between 15%-50%, damage = 2, rate of attack = 1 per second
below 15%, boss enters a frenzy state, damage = 2, rate of attack = 3 per second

The boss might even have different kind of attack at each band. The idea here is that the boss becomes harder to beat as the battle goes on and the player has to adapt their strategy to beat it.
I hope this gives you some ideas