The best powers for the bosses are:

for bosses 1 and 2 it's fire,
for bosses 3 and 4 it's boomerang,
for bosses 5 and 6 it's tornado,
for boss 7 it's bombs,
for the final boss{part 1 }it's tornado again,
for the final boss { part 2 } well the only weapon you get, the staff .