How to show that a problem is easy?
You are right. Any problem in $\mathsf{P}$ can be (polytime) reduced to the halting problem, for example. (In fact, any problem in $\mathsf{P}$ can be polytime reduced to any non-trivial language, that is any language other than $\emptyset$ or $\Sigma^*$.)

What is true is that if A reduces to B and B is easy, then so is A. In particular, if A polytime reduces to B and B is in $\mathsf{P}$, then A is also in $\mathsf{P}$.