Knowing DS is key as a warrior to do IB, specially after Aetherial Profusion/twister phase.
Like someone mention, IB is on a GCD so must prematurely use IB before DS is been cast/last till is cast. or you wait few sec ~5 at most just waiting for it.

DS cast is about every 30-32 sec (can change if twin has to fireball ect.)

to know when DS will hit I setup a macro.

Macro "Brutal Swing" with a party tell on DS (I have no idea what the command is now lol)
The way I have is /wait 24 sec
/<se1> DEATH SENTENCE IN 5 SEC
So every time Twintania uses DS, USE "Brutal Swing" off-GCD, this will start the DS countdown and will give a 5 sec warning to party chat at 24 sec, giving YOU and HEALER a good warning so u can use IB.
Your "PF" healers will love u for this macro,