How did celestia use the elements against nightmare moon.

[pieisgood 0]

if the mane 6 and the pillars hypotheticaly needed  to  use all of the power from the tree for a banishing spell how did celestia banish nightmare moon just using the elements not sacrifice them? 

[PoisonClaw 8,134]

In the episode. they state that because the Tree of Harmony is still recovering from the Plunder Vines, using the Elements could potentially use up what little energy the Tree has in reserves. That's why using the Elements on The Pony Of Shadows was so risky, since a spell to banish something to another dimension entirely would require a huge amount of energy.

When Celestia used the Elements on Nightmare Moon, their link to the Tree was presumably still strong since it had only been a few years since they had been removed from the Tree to use against Discord and the Tree still had boatloads of energy in reserves. Plus, the spell Celestia used seemed different than the banishment spell Starswirl intended to use, since Nightmare Moon was only sealed inside the moon for 1000 years and not sent to another dimension.