Our ele shammies and boomkin knock them back. Our boomkin also does some meter padding with dotting them up which is semi-useful since a bunch die, melee also cleaves them if they come close or they get some aoe that's going for the fragments. But its mostly the knockbacks.
Not sure if you're doing this already but you can ignore adds after the 3rd spark dies and just focus rhyolith down he should be around 40% or lower by that point. Can use BL if you have trouble pushing it.
Sounds like they are not getting slowed. For us between the 2 enhance shamans totems and 1 frost dk they stay perma slowed... then we have 2 elemental shamans jump in for knockbacks from time to time. They also use flameshock on cooldown and spam fire nova, but mainly that's cause it looks fun on the video recaps... I'm sure it helps a bit.
We'll switch to burning boss when he is around 50% health and just as a sparks spawns and the current fragments die. That way your not at risk of having a fragment blow up. Lust and burn the boss and it's not a very tight dps race at all with that timing. But your stacks should be fairly low by then.