Based on a lot of experience teaching and tutoring I'd say (for good mathematicians who exit gcse with excellent algebra and trig skills) .......
C1 - easy, mostly A/A* gcse content with an introduction to calculus. Never the less, Make sure you study this unit thoroughly to get a solid foundation to build on.
C2 - a bit harder than C1 but not much, applications of differentiation and trigonometry seem to cause the most grief. Not sure why but logs seems to be a bug bear of many students.
S1 - easy, mostly gcse. Parts of the probability are tricky and normal distribution hurts until it clicks
S2 - straightforward if S1 was ok. Lot of new stuff to learn but none of it hard.
D1 - easy content but fiddly questions in exams can be prone to errors and time consuming
D2 - no experience
M1 - mostly easy content especially if you've seen the equations before in physics, vectors can take a bit of getting used to especially if they are rushed at the end of the course. Exam papers are frequently and needlessly tricky in my opinion. Exam practice with recent papers is vital as is thorough work on the mixed exercises and reviews in the official text book.
M2 - new stuff to learn, harder than M1 but not hugely difficult if M1 was ok.
FP1 - harder than C1/2 but not much and easier than C4.
FP2 - hardest of the units you listed.
Rock solid skills in C1 and 2 are essential to success in C3&4
C3 - logical progression in core maths study. Big focus on differentiation but with plenty of practice and a look at the formula book you realise there's not a great deal to memorise. Trigonometry causes the most problems but again their isn't as much to learn as you think. If you find memorisation easy then learn all of the identities. I prefer to learn how to derive them with reminders from the formula book. Exam papers have increased in difficulty recently but only in that they test knowledge with unfamiliar questions. This works against candidates who lack a thorough understanding and have relied largely on smashing lots of past papers.
C4 - lot of easy content building on c2 skills but the integration chapters are a killer. Practice, practice, practice and more practice required.
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