No AS is really not that hard if you study from good teachers. A levels i very overrated. A2 is a bit hard but even then if you study from the best teachers, you will get good marks easily. I studied from Naushad (Best physics teacher in karachi) and i got 95% in my A levels in Physics .
And you'd be suprised to hear that i was terrible at physics, i gave AS paper in June and i think i got 76 or something like that then i decided to give AS paper again in December and i got 91 in AS and then i gave A2 paper in June and i got 95% overall in A Levels.
So if you think about it, its really not that hard but you have to work consistently and you have to study daily and make sure you take tutions from the best teachers.
For A Level maths, go to Aver (Defence) to Sir Shafqat, very good teacher. I got 90 in Maths A levels. I really liked his style of teaching, some people dont but take a couple of trial classes from him if you like him then study from him. The best thing is, he teaches individually and also in classes so if you want to study individually, he can do that for you.
For A Level Physics, Go to Sir Naushad. Very famous teacher in karachi of physics. Teaches at various institutions so you will have to find out about it because he doesnt teach at the center at which i took his classes anymore. But you MUST take classes from him. Physics will be so easy then !!
For A level chemistry i cant recommend as i never studied it.
Oh and, in AS they teach you pretty much from the basics so dont worry O levels will not be applied alot anyway so dont worry about forgetting things.