He's an incoherent mess because Togashi thought he was cooking with the 7 personalities, it really just makes each version of him dumber than the last.
"You don't stand a chance against me NOW Yusuke, for I am Personality #2, and I am the STRONGEST!"
"You really don't have a chance, for I am the TRUE SENSUI, Personality #4, the STRONGEST!"
Repeat until Yusuke unlocks his literal demon king DNA that pushes him beyond an S-class fighter, so far that Hiei says Sensui has no chance against this edgy shitty power up.