I've only just managed to beat the Shinobi Hunter and got the beads to upgrade my vitality and that certainly makes a difference. The purple guy is my next target in the bamboo thicket. Yes, the exploration and choice of route aspect is great but, like you, I'm beginning to tire of the specificity of these boss encounters. Two await ahead now - I've reached their arenas but stopped short of triggering the Blazing Bull, and the Horseman.