Note: The other classes I haven't played so I can't
comment on them.
But here's the general strategy:
POW tree = powerful, you'll do heavy damage but
take more damage than Dex. This type is good for
PK.
Dex tree = tank, you'll do lower damage but take
less damage than Pow. This type is good for
leveling.
Int tree = AOE, you can attack a lot of monsters at
once for heavy damage and you can cast magic fast.
But in PK, if the enemies have a lot of magic
resistance then your spells won't do a lot of
damage. This type is very good for leveling as
well.
First pick a tree to specialize in, POW, Dex, or
INT and decide you want any other skills from
another tree (Usually a passive skill that
increases damage).
Level the skills just enough so that it fulfills
the prerequisite for the next skill. The reason for
this is that the starting skills are usually weak
compared to the final skills. But there are some
exceptions to this rule: You generally always max
the passive skills and there might be an early
skill that you will end up using even when you
learn other skills such as archer's speed shot. You
will be using that skill forever! So the best thing
to do is to find out such information before you
make the character or you'll end up having to use a
Water later on! They cost a LOT, just trust me.