Teng
15
It turns out that in this case, as in the old purge, archers can be useful for distracting attackers if they are placed further away from the gate, on platforms.
I tried this structure in eight purges of different levels, and I like it.

Archers fire at the approaching squad, and more than half of the attackers run towards them, under platforms. During the time they crowd there, the defenders manage to kill some of them without much harm to themselves. The result is a good relief for the defenders, and they save a little time for healing.
These same archers kill Stygian archers if their base appears nearby.
In each pair of archers, one shoots gas arrows, the other shoots star-metal, but most likely it is better to opt for gas arrows.