WHM can also Heavy with Stone to help you gain a ton of ground on the boss, and it actually lasts a very long time.
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... if you're smart you can estimate the time it takes for him to use flail and fluid aura the boss away as well without all the running. As a side note, he tends to use fire before he uses it (if you're lucky) and you can just LoS the ability by running behind as he casts fire and he'll aim for where you were before moving when he finishes casting and does flail.
Was going to mention the same thing, watch the fire spell and run behind. Personally what I've done is wait at one corner of the room, let it come to me, then wait for the fire spell to go start casting, then run to the side (to break line of sight to cancel the cast) and just run to the other corner of the room. This has worked for me (as well as Sprint + running along side the edges of the room).
I have taught many a tank how to dodge the spell casts by running behind. It never hurts to try new things.
Never got hit.
You just need to cast Sprint immediately and run around the battlefield, making the boss lose your tracks. Run until you see the boss doing the flail.
Sprint is REALLY helpful, people underestimate it.