Strašen can be used to:
1. Upscale the word
Example: Strašno vroče mi je bilo - It was terribly hot (to me).
Example: Strašno rad je vozil avto - He really really liked to drive a car.
2. To express scariness
Example: Imel je strašno masko. - He had a scary mask
Example: Prestrašen je prišel domov. - He got home frightened
Grozen can be used for:
1. Expressing awfulness
Example: Izgledal je grozno - He looked awful.
Example: Grozen si - You're awful
2. Upscale the word that usually describes awful situation.
Example: Grozno vroče mi je bilo - It was terribly hot (to me).
Both has similar feel but is slightly different in use. I would associate groza/grozen with terror from awful situations that have good reason to feel that way and are stronger, while for strašno/strašen I would associate with scary situations and may not always have good basis or excuse to be scared, strong but maybe not as strong.
One could argue that they mean the same thing but have different scale. Similar to being scared and being terrified.
But for example if you say: "Grozen si" it means You're awful , while if you say "Strašen si" - You are scary. In first case you could be scolding person but are not necessarily afraid of person. In second you could admit that person is too powerful or crazy and you're afraid of that person as result.
So it's not easy to simplify usage of those words. You just need to hear use of those words many times and you will learn by experience. I don't think there is any good rule but I could be wrong. I think ways I presented are general ways that those words can be used so I hope maybe it helped you at least a bit.