edit OP, disregard this answer, it does not apply to your situation, read my next post in this same thread.
If you are in this game for the sake of being able to solo everything, bad news for you in regards to r8 content.
You would party to run through it, unless you have scout. Scout makes some things LESS harder, not that it becomes easy. Cloaking is the reason of why it makes some stuff LESS HARDER.
SR is good because it has good aoe dps potential with some good single target as well. It shines in the DPS department due to Retreat Shot and Assault Fire. If you want more elaborated answer, the only way to do it is simply answer that it has good damage.
SR requires some ability to perform, you will not aoe dps as easy as it would be to an elememe or doppelsoldner (although doppels do rely on their own ability to avoid or take damage).
But it is, indeed, the aoe class that has the easiest time soloing stuff due to how retreat shot works and how it can be used in solo scenarios.
Constant damage, yes, they surpass it in the aoe department. Although a mergen can make it better if they can unleash triple arrow twice and there is a group of monsters JPed, but they would need to be of the flying type for that to happen.
Constant damage wise, flecther3mergen is better than SR for single target that is.
And if i were you iw ould just reroll. Reaching r6 doesnt take that much time, to me it takes like, 3-4 days and there are better ranks than a3 and wugu2 for both SR3 and Flecther3Mergen.
But if you are not in the mood, just move on and choose sr3. Flecther3Mergen cant be put into your build.