Nail these ones first, you will need the skills on the rest.
Unit 4 Fractions and Percentages
We are going to apply the algebra skills deeper into other topics now.
Unit 8 Perimeter, Area & Volume
Problem solving skills to the fore, with a great big dollop of multiplication.
Unit 12 Right Angled Triangles
Unit 14 Multiplicative Reasoning
If you are pushing for a grade 5 then these are your nemesis. Ensure you have skilled up on the previous units.
Unit 17 Perimeter, Area and Volume 2
Unit 18 Fractions, Indices and Standard Form
Unit 19 Congruence and Vectors
Use the PLC below and focus on Units 1-5 where you are nope or Bad. Then do the same with 6-10 and 11-15. Personally I would stop at 15 unless you are going for a grade 5. When you are OK at 1-15 then it is worth doing past papers and problem solving.