GCSE Combined Science is not designed for lower attaining students. GCSE Combined Science assessments are designed to test the whole grade range from 9-9 to 1-1. The standard on Combined Science is the same as separate sciences, it is not easier.
If you have a student who is better at one science over the other sciences, it may be worth thinking about entering them for separate sciences. Then the weaker sciences will not affect the grade of the one they are good at.
We have produced a fact sheet to provide information to help you with these decisions: