for me i think chemistry was pretty manageable in the O
levels
but in the A levels, i think chemistry is much much tougher
for most of the topics it's really memorizing (e.g. Organic Chem,
Periodicity)
but for those topics involving calculations and understanding, i
suggest you read up and eliminate any doubts about them (e.g.
topics like Ionic Equilibrium, Chemical Equilibrium). this way, you
can answer any questions they throw at you since you understand the
concepts well
but most of all, practice and practice and practice and practice
and practice...
you get the idea 
