1.Which skills is good to dont replace? And why?
2. DualSword/DualBlunt? And which SA and Element is best?
3:Robe/Light/Heavy and which type? (Attack, Defense, Recovery, Magic type...)
4. Which harmony is best for me in solo exp, PVP and party PVE?
1: Not sure about Delf
2: Dual Swords are superior crit wise,most common SA for ISS is health as for element all elements the only 2 ones you might wanna avoid are dark and earth,cuz they are bad at pve.
3: Heavy attack type.
4: I use Warrior when solo exping,Knight when grouping,as for pvp i suppose Knight aswell,i havent get to pvp with my ISS lol.
In a full group consider wizard's harmony because you can keep your debuffs up more (20% decrease on skill reuse time). Experiment and see if your DD's are well geared and you're not, your party will do more damage with keeping up the debuffs than through your weak Iss crits.
If you have bad armor, Knights, so as not to die or need heals all the time.
The best thing about being an Iss is that you can change your harmony at any time. And if you're running too slow you can alt+click off drum melody, gain 20% speed then rebuff at your destination.
For duals, depending on server, swords may be significantly more expensive. When soloing you can avoid crit rate being an issue by spamming skills. In parties, again unless you have boss jewelry your crit power isn't that great. And I think one of your buffs reduces received crit damage, so keep that in mind in pvp. Go for the best you can afford, and then don't blow them up trying to over-enchant like that guy who's so "lucky" (with his credit card).
(And if anyone sees my Iss posts and wonders why I address how to play as a poorly-geared Iss in a poorly-geared party, it's because that's the people who post on the forums. No use in stating the ideal, or looking for 'good' parties, when their reality is having 0 element armor and 30m in their inv.)