Uuuuuuugh Leviathan...I hate this because it's the camera that can *** you over..
Don't have to worry too much about positioning on the HM version since you can't fall off.
One thing to note is his reflect damage buffs on the tail and head. Tail reflects magic damage and the head reflects ranged physical (brd/mch). If you're melee, you're fine to attack either.
I typically keep my camera zoomed all the way out during any fight though. Especially helps on Levi.
Now Levi EX is a different story, since you can fall off. But these days, burning him down too fast is a bigger issue than falling off. Due to how it can cause him to skip phases, which makes his ultimate ability use erratic. (Unless you just go with like 3-5 people)