You don't need to cast your own timestop to be able to move during the enemy's timestop. All 4 of the special weapons let you move while time is stopped, whether you or the enemy casted it.
That said, it's pretty necessary to have a few of them to survive deep LoC reaper. I hear deeper Nuruko boss can stop time, too, but never saw her do it myself. Never gone past floor 2000, though.