This boss is surrounded by minions to protect itself. If that wasn't enough, once a minion has received a certain amount of damage, it detaches itself of the formation increasing the number of enemy units on screen.
Strategy 1 - Using a weapon like the default fireball
These fireballs are generally destroyed when hitting an enemy (power shots may get through an enemy if they take it down, but not as power shots any more). This forces us to deal with some minions before we can concentrate on dealing damage to the boss. Instead of shooting indiscriminately we can concentrate on one side of the formation so the boss is vulnerable from there, at the same time we can deal with any detached minions that may be bothering us.
Strategy 2 - Using a weapon that can go through enemies like the shuriken
A shuriken is not destroyed when hitting an enemy, so shots directed to the boss will damage it from the beginning, the main danger being the shuriken also damages minions in its path. If you are not careful you could end up with the screen full of detached minions and you may need to take care of them too.