step 1 : get a necron that can stun and remove armor
step 2 : wait for stun and armor removal to be applied then missile -> auto attack -> missile etc
step 3 : profit

the necron build :

the tau build :

for extra safety, 3rd player can also be a necron to provide a second stun orb.
at the end of wave 15, let the 3rd player kill the last enemies so both the tau and necron can be ready at the clone spawn