Pre-casting with Mirror Blade is quite free, so I'd advise people to do it here.
Just inch a bit forward until you see that the skill is in range on your skill bar, this way you will avoid pulling Gorseval by accident.
For the split phase, you want to cast your phantasms on the next spirit that your squad will go to.
This is because you want to benefit from Chronophantasma as much as possible, which you won't do if everyone attacks it and kills the spirit too fast.
Lastly, you can still pre-cast the clones for the upcoming phase just beware that they will despawn after six seconds IF Gorseval doesn't become vulnerable within that timeframe.
-- Mesmer Resources --
The spreadsheet contains rotations, skill damage and more!
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