which skills in any archer tree do you think is worth to have 100 enhance attribute?
Neither.
The cost of enhancing from 90~100 is way higher than other methods of enhancing atk.
If you really MUST pick, then Magic Arrow ,Retreat Shot and Split Arrows. They are the core skill that define a certain spec.
so 90 attribute is optimal enough between cost/benefit ratio?
multi shoot , because cheap
Lol I have this on 90. MS reallly good if you have chrono in party.
Oblique shot 100 attri too, very useful for me.
what lv are you now? 330?
Yes 330 fltcher 3 mergen, so without twin arrows or split shot. With a good transcended bow I usually farm lvl 145/130 with ease 18/15 mins only using oblique shot lvl 1 and 2 mana pots.
please define: good transcended bow
Trans 6 aufgowle bow +8 
do you always 100% transcend it or gamble using below 100% to transcend it?