Generally, avoid making sudden movements and staying near the bosses helps a lot especially with the kangaroo and snake. You don't have to throw your boomerang at max speed, since it will return to you at that speed too. You want it at around the same speed as the boss is moving so you hit it multiple times as its coming back.
all my friends irl also said this game was pretty hard, but once you get the feel of it I think you will find it quite fun.
good luck!