Leviathan ex is not that hard but i have sometimes issue

[Remn]

the trick is, after the first slam, you have to place youself on the side that levi will slam. this will allow you to atk him while sliding (same for all class you can still atk the boss if you are in range. with any instant action)
doing this, you can use fast blade on the boss, then run over to where the tail spawn and do savage blade > RoH, the first set of add should spawn after RoH. at this point you will have enough hate for a while.
after your malee add die, you should have about 10 more second before levi jump (provide that you group unable to push levi to skip jump/drive), use this time to build more hate. (you should be able to do at least 2 RoH combo)
once the caster pop, you hate on tail should be high enough.

the trick is your team need to focus your add first, that will give you more time to build tail hate
if you still have problem, you can drag your add close to tail,
then do, FastBlade > Savage Blade on Add and RoH on Tail (just tab target between the 2)

if it still not working, check your buff , you may forgot Shield Oath lol