You can never really tell because there are people who backstab or make their own betrayal on their allies, but it would be hard because, like you said, the security would be there and the assassin that was hired won't get near Trump easily.
For terrorist organizations such as ISIS, it is simply not worth the time and resources to assassinate Trump. At the most they have less than 0.000001% chance of succeeding. And for that they will have to sacrifice their best men and resources.