As an engineer who hadn't studied that type of math in quite a while, Elements was pretty tough and I was getting stuck a lot.
ISLR introduces you to many of the same topics in a less rigorous way. Once I was familiar with the topics and had worked through the exercises, Elements became much easier to learn from.
If you reading Elements is difficult then I would recommend Introduction.
I'm not sure if reading Introduction will prepare you for Elements so much as it will just give you some knowledge you can use and see if it makes sense for you and what you want to do to go and (re)learn some of the math tidbits that you need for Elements.
