I took me qute a bit to down this as MM, more then in Legion (and I did it before Antorus patch) since our Legion artifact ability was really handy for the mechanics.
Also, it is sickening how much easier this fight is for a frost mage, their entire toolkit comes together for this fight - constant aoe dmg/perma slow funneling into the main boss (frozen orb ftw).
Some tips which worked for me:
During the first Karam phase, kite him on the edge of the platform, you can try to do this in the second phase as well, but be careful not to miss the hand add and interrupt. This is so he drops puddles far from Raest.
Altough the guide states to use BL on start of ph 3, ive found it was the best to use it when Raest first becomes attackable - together with pot, you should aim to reduce Raest the lowest you can - unlike many MT fights, this one gets progressively harder as you prolong it.
I went full single target build - that means Careful Aim and LnL. Ive only really cleaved the adds on first two intermissions, the rest is just not efficent. Honestly, the number of adds i had at the end was meme lvl.
Put Karam on focus and use the concussive shot macro for him. Remember that staedy shot increases the duration, so in the first two Karam phases. He can also be trapped and binded - dont just use them tho, especially when he gets faster, time them before hands + runes. You can also knock him with Bursting, but be really really careful with that, the hitbox can be easy to mess up.
Keep tar trap for the adds, but they shouldn’t be the problem in this fight…they are slow.
Start the fight without your pet for the Lone wolf buff, summon him either on second Kram phase, or even third if you are comfortable. But later he must be out for the runs - it makes the fight a ton easier. Make sure to macro /move to for your pet and send him immediately to the spot - I’ve noticed pets sometimes dont go straight for the rune, so this is important. If the rune does go off, the add can be trapped for the whole 30 seconds, but it really shouldnt happen, as you need it for Karam.
Dont use Master Marksman - it can break trap on Karam. Either serpent sting or even crows (which are good for hands too because it will reset).
Good luck and dont be dissapointed if it takes you more then 50 tries - the fight is brutal and there are 3 1 shot mechanics. Honestly, 4 becuase even a small pack of adds which got into melee range will obliterate you in 2 seconds.