How should i balance my monsters

I have created a few new classes and races for my game they all have the strengths and weaknesses but are still very powerfull.What is the best way to balance monsters or if I should simply put in more powerful creatures.My players are currently playing my campaign were they are traveling to the astral plane to kill an empyrean as a favor for having a devil save them in battle.I don’t want to keep throwing out powerful creatures but don’t want them obliterating all the weaker ones so how should I balance these monsters either give me tip on the best way to balance monsters or send me custom rules and monsters that can be fought at a medium difficulty.This game is in 5e