Ok, I'm gonna take a poke at this. Since this thread is talking about why people suck at Hashmal and not necessarily how they're sucking at their job, I'm just gonna put how my first time went. (went in blindly as my smn)

Quake IV

Initial reaction: Don't see any telegraphs so I'm gonna assume its some tankbuster or raid wide dmg
Final reaction: Ok, raid wide dmg. not a prob

Jagged Edge
Initial reaction: Ah, its the red poop of death with delays. Ive seen this before
Secondary reaction: And people are dying to that. Guess they haven't experienced that mech too often.
Final reaction: Guess it invisibly targets some people since it continues a couple more times and not necessarily where I am.

Rock cuttter:
Initial reaction: Haha, a pokemon move. I think. Ok where's its coming from now
Secondary reaction: nothing came up. hmm.
Final reaction: *after seeing it like, 5 times in the raid* didn't hurt nobdy else in my party and my tank's not MT. Maybe its a tankbuster

Extreme edge:
Initial reaction: Oh shit, its one of those dragon boss dive thingies. I'll just move to the left---aaaand everyone went right. I'm probably gonna die. (dies to it)
Secondary reaction: *After dying two more times to it* When it doubt adjust camera. *Spins camera like, everywhere until I see Hashmal* Ooooh
Final reaction: Still died to it cuz I was too slow at running. Maybe there's a visual clue about this guy. *Would later check in a guide that the arm that burns is the side you don't want to be into*

Earth Hammer:
Initial reaction: Ah, the ole meteor wide thingie. Run to the edge for dear life.
Final reaction: Easy peasy

Control Tower:
Initial reaction: *after seeing Hashmal cut the pillars* that's pretty badass. *dodges them
Secondary reaction: Huh, its actually easier to dodge these if you're close to the boss cuz you have an idea of where they're falling. Plus I can run between them.
Final reaction: *After seeing lots of people die to it* Visual based mechs are gonna be the bane of everyone's playthrough this raid.

Command tower:
Initial reaction: *after seeing someone kick the bucket from it* rule of thumb: no one should ever stand above or under big things. Unless you're bismarck.
Secondary reaction: Ah, that's our dps check. Cross the fingers and hope everyone knows their basics. *Sand spheres are summoned* Oh. Basics are not gonna cover this. Better haul ass.
Final reaction: *After clearing the dps check* Ok, I have hope. Now if I stop dying to Hashmal's dive, everything will be peachy.

Golem Summon:
Initial reaction: Moar dps checks. Yaaay
*Final reaction: I love being a smn


So yeah that was me on my first try. The hashmal dive was the one that gave me the most trouble cuz I couldn't pick up on his visuals.