My suggestion is to remake the basic layout of the whole thing. Specifically - to make several parts with each one boss at a point which is not avoidable and some unique behavior for every area. For example the first area has some regular bullets and the boss has regular bullets as well, just plenty more. The next area could introduce the homing attacks and same thing with the boss. After that the area mixes both types, the next one introduces a new type, and so on. The final boss is the one where you actually have to show mastery of everything the game has to offer at once.