by editing patterns.txt you can mod the boss's attack patterns.

despite the misleading file extension, patterns.txt contains lua code.

each pattern consists of a set of shots, for which you must define a time t, a direction dir where math.pi/2 is straight down, xoffst and yoffst offset from the boss, and bul, the bullet type. 

then boss.patterns consists of a series of tables each representing a stage. during a stage, the boss randomly selects patterns from inside the stage.