So i don't hope this is a re post, but my guild did Flame Leviathan yesterday.
A little into the encounter he changed his follow target as he normally do, only thing different about this time was that he disconnected around 2-3 seconds after Levi started following him around.
This caused FL to stop dead in his tracks and stand still for the rest of the fight, he didn't evade bug or anything, he still cast Flame Jet as normal and all the other attacks, but he didn't move at all through the rest of the fight.
After we cleared 10Man we tried it on 25Man and it worked as well there.
I hope anyone can find this useful.
So this is what we did.
1. Wait for him to follow one of you members (or yourself ofc)
2. Fake a DC pull you internet cable, whatever needed.
3. Rest of the raid kills him of with ease, though hes not hard anyway.
the player who dc'ed can just join vent or TS or whatever you use and wait with logging in till the raid killed him, remember not to loot before the DC'ed person is back online.