Would you like an in-game parser?

[Reynhart]

What? I'm being serious here. It's like what you said, if its the mechanics, you see what you need to fix, if its your rotation you simply can't go back to the stone, sky, sea and clear it till its easy.

It depends. If your rotation has a clear flaw, yes, you can. If it's your rotation, in a real situation like Ghislain described, then I'd ask the same question for you.
What would you do for such a DPS ?

As for your example for Sephi EX, I don't think it would really work either. SSS numbers are actually higher then real fights, since you don't have to deal with mechanics. But meeting those numbers are also not a joke. So, I'm pretty sure that a DPS who can beat SSS for Sephi EX, and that doesn't screw during mechanis will have no issue meeting the DPS requirement.

Yes I did train my DRG rotation without parser too but did I see how good I did in primals or savage? Nope, when I found out, holy crap I was shit. And I talk about really bad.

Believe me, the first time I did SSS as a DRG for A1S, I realized how flawed my rotation was. Then I looked at Dervy's guide, completely changed my cross-hotbars and practice the rotation time and time over. And when I get back to SSS, the difference was HUGE.
With no parser.