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. Doesn't work like that. Just because you beat the dummy strike doesn't mean you are on pair with mechanics and dps wise. It's like putting a blm on a dummy for seph extreme and they clear it easy then put them in the fight where you have alot of movement going on; shakers, colours, spread out etc. You can't excactly expect your dps to be the same which is obvious but the fact you need to work around your dps rotation around the mechanics is something that you need to do. 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. You can't really compare SSS to a fight with real mechanics. If it was that simple you would see mnks would be top dps in any fights according to your logic.
I know this wasn't for me, but I'll reply to you on this matter on my behalf. If I see some one using my class I would ask them when they do their rotation and when they use cds, do they hold them or use them right as they come off, because not always its a good time to use them. Example seph ex, b4b pops right as the hand fist pops and you wouldn't pop b4b as hes about to stomp you away, because you wouldn't get max benefit of b4b. I would ask what his rotations and see what is wrong, then ask when he does debuff and actually put dot up on full time. There is a lot of thing that comes to his class but what most can't handle is doing mechanics while doing skills which makes you lose uptime on the boss: You dont dot the boss for example before you can't hit it, like in seph ex you want to have both dots up before he smashes you away. You want to do piercing debuf before he does it as well so when you come back from the hand smash you have debuff and it increases your dmg when you get back to it and also the dots are still on. There are small things that matters in this fight. Line up your skills with the mechanics so you don't pop them wrong them etc. I would tell them in details and when and how. NOW, if this guy was playing on a class I have no clue about, I would send them to my friends who can and this has happened quite few times and goddamn, as mentioned. This guy increased his dps on a dummy from 900 to 1.4k. Then on primals it went from sorry 900 to 1.3k. Why? because he listened and took advice and tried to do the rotation and danced with the mechanics while doing good dps. A dummy is a dummy, most people can pull it off. Even if I did midas 4 savage as 220 drg with 220 spear, doesn't mean I could pull enough dmg on the actual fight. Mechanics, rng, if you get a certain mechanic and you can't hit it, example when you have to go away from the group when he jumps on someone, you have to move away. There is a lot of factors that comes to place. Now if he took my advice and improved and I clearly see its a improvement, good good, he stays and he gets better. But when you don't get better after 5-10 rounds and still do the same amount as before, you clearly didn't take the lesson proppely or even serious. Doing a fight as a dps and have high dps isnt all about clicking 1 2 3, it's matter when and how and why.