Arcane mage opening, 6+ seconds of no damage.

[Marrilaife]

Unfortunately, this new tier is almost entirely cleave fights..

Huh?
Except adds on first boss, Jadefire Masters, Rastakhan, and maybe Jaina (it's more aoe than cleave in last phase), which exactly fight is a cleave fight?
Opulence has a wave of adds (that should theoretically be ok for Arcane same as Aluriel was in Nighthold).
Grong, Council of Loa and Merkatorque have all "anti-cleave" mechanics in built (not supposed to stack Grong with robot, not supposed to stack Loa plus they heal if 1 dies a la Botanist, Merkatorque robots can't be killed except with the special buttons).
Stormwall blockade theoretically at some point you get double / multiple adds and you could stack them, but I don't know if they're bugged or what, every time we tried it, one of them would randomly blink forward (wtf... I never heard of this being intended but it's happening a lot in the raid).

There are several mechanics where random add spawns that needs to be killed, but that rather prios multi dotters than cleave (xuen on jadefire masters, robot on grong, raptors on loa, mariners on jaina) since these adds aren't stacked with the boss or roam around. I expect spriests, boomkins, and aff / destrolocks beat mages in these cases, no matter the spec of the mage.

[TyrianFC]

Stormwall blockade theoretically at some point you get double / multiple adds and you could stack them, but I don't know if they're bugged or what, every time we tried it, one of them would randomly blink forward (wtf... I never heard of this being intended but it's happening a lot in the raid).

Splitting Ice is very good on Blockade - starting from ~70 seconds into p2 when second add spawns. At that point its amazing and the adds are often close together to cleave (esp if your raid has a good way to handle it). With +50% dam taken from kelp, my glacial spikes could crit for over 200k, in addition to the 80% split hitting another add.