Firstly, if you want to do Medicine, you really need to think hard about it. You need to focus on getting into Medicine, not Cambridge specifically, because it's hard. As well as good grades in GCSEs and A-Levels and a good BMAT/UKCAT performance, you need to have work experience, voluntary work and generally be able to show why you want to be a doctor and why you'd be good at it. So first of all I'd recommend you get some work experience, do some reading and decide if you want to do Medicine or NatSci, rather than how best to get into Cambridge.
Now, with regard to your questions:
1)What A levels would you recommend to take? For Medicine at Cambridge, Chemistry is essential, and it's compulsory for lots of other places too. I'd also recommend Biology if you want to do Medicine anywhere, and maybe one more science (there are a few colleges at Cambridge requiring 3 sciences still but all other unis now only require 2). Other medical schools e.g. UCL like you to have a humanity/arts subject though. If you want to do NatSci, again you'll need Chemistry, plus whichever sciences you're more interested in. I'd recommend doing at least one Maths; two if you want to do physical NatSci. There is, however, no need for Further Maths if you end up going for Medicine.
2)Is the following a good combination for A levels?
Biology, chemistry, mathematics, physics(only up to AS), and a language(something like german or chinese). Yes, if you think you can cope with (by which I mean get high As in) all 5. Otherwise ditch one of the sciences, unless you want to go for NatSci, in which case ditch the language and do more Maths.
3)My school requires everyone to do an AS level in critical thinking or general studies. Which one is a better one to do? And in addition, is critical thinking suitable for someone (who enjoys english the least) to take? I doubt Cambridge give a monkeys, to be honest. I took neither. If pushed, I'd go for CT as everyone seems to think GS is a doss.
4)How much work experience is one advised to have? See the Medics forum for more information on this. It's as much about quality as quantity. But Cambridge care less about this than most other places, from what I've seen. As for NatSci, I have no idea.
5)How intensive is the medicine course? Is it the hardest course out there? It's the hardest Cambridge course I've ever taken.
Seriously though, it IS hard work, especially 2nd year (generally - or at least among my friends who I've terrorised - acknowledged to be the hardest year of any Cambridge degree), but it's tolerable. And I wouldn't say it's MUCH more than most other subjects, just different.