I did AQA A Level Maths and ended up with an A which I was really happy with since maths isn't my strong point. The AQA board is quite easy overall, it's easy to get an A by doing past papers because they're starting to run out of ways to make original questions, but A2 papers always have "stretch and challenge" questions for A* students which you can never predict. Here is my opinion on the overall module difficulty:
C1: Very easy module since it's just a recap of GCSE with a few new topics which are quite simple e.g differentiation, integration, circles
C2: Slightly tougher module. Some GCSE recap topics (trigonometry) but things get tougher with trig identities and solving trig equations. Logarithms seem tough at first but keep doing past papers until you're confident - they can't test you on anything new with them. Other topics like sequences, series, differentiation and integration are simple.
C3: Hardest module IMO. I got A*'s in mock tests and ended up with a C, god knows how. Packed with difficult differentiation and integration which takes a little longer to take in and understand (particularly integration by substitution) - again, with past papers it's not such a big deal. More trig graphs + trig identities but nothing you haven't seen before. Modulus graphs can be done with graphing calculators. Functions can be confusing, exponentials and logarithms is easier than C2 logs i feel.
C4: Harder than C2 but not as hard as C3. Lots of new topics which rely on C3 differentiation and integration techniques but these are easy, apart from cartesian equations which can be tricky (just do past papers q's). Partial fractions and binomial expansions easy.. Vectors are very tricky, easy to pick up a couple of marks but you need to thoroughly understand them to get high marks which requires you to be logical and able to draw diagrams etc. Exponential growth/decay is just C3 logs/exponentials applied in context but these questions can get tough if you aren't masterful in logs. Lots of new trig identities and formulas to learn, trig proofs can be tricky. Differential equations are not hard but you need to be good at algebraic manipulation and integration.
D1: Very easy module to get a high grade in since you're applying the same algorithms over and over. Difficult topics include linear programming (graph drawing) and graph theory.
S1: Another very easy module, you just need to remember prelearnt answers and know how to use a graphical calculator to answer every different style of question, which becomes simple when you've done the past papers.