I am the only person that works with my boss and he knows some things about the stuff I work on but is not an expert. Sometimes he tells me to do something that he thinks would be easy but is actually not trivial and when I don't get it done I sometimes get blamed for it. How to deal with this and let him know this something is not trivial to do like he thinks it is.

I am the only person that works with my boss and he knows some things about the stuff I work on, but he is not an expert. Sometimes he tells me to do something that he thinks would be easy, but it is actually not trivial and when I don't get it done I sometimes get blamed for it. How can I deal with this and let him know this something is not trivial to do, like he thinks it is?