ALL OF US WILL GO TO THE PARTY, "LESS" MY BOSS
The only real way that you are going to be able to stop making mistakes like this is if you expose yourself more to English! When a Spanish speaker says a sentence like this, they are thinking in Spanish either consciously or subconsciously about the word "menos". That is why I hear this mistake quite often.
"Less" is not possible though and the correct word is "except". So you need to say: "All of us will go to the party except my boss".
Hope that helps!
Soy el director de Daniel Smith + Partners. Tengo más de 10 años de experiencia enseñando inglés a españoles.