The strategy here, which can be reproduced, is as follows:

• Place the troops far enough away so that they can attack for the most part with spears, which do far more damage.

• Save a fireball for last, to finish as quickly as possible. What's more, it needs to be positioned in such a way that the last troops pass over the embers left by the spell.

I think you could easily save 200 to 300ms on the speed at which the last wave passes and the archer release window appears.