If I understood the scene correctly, the whole point here was that LTT/Rand knew that unlike Rahvin or Sammael or the like who were arrogant and such, Graendal would never face Rand in a direct fight.

Using your idea, the moment Rand attacked the palace, Graendal would open a gateway and flee away, avoiding the battle entirely.

That is why Rand did what he did. Because it was the only way to take out the slippery Graendal. Direct confrontation would simply not have been possible with Graendal since she would always have easily escaped with traveling.

The only way what you suggest is possible, if Rand could have somehow warded the area in such a way that Graendal could not open a gateway to escape. However I don't think Rand knew this technique yet. So it was either balefire the whole place, or let Graendal escape. Those were sadly, the only two possible options.