LOL

technically, in game, Archimond takes no damage from wisps, when they cast detonate, because even though he is summoned, he has spell immunity, so even if a hundred wisps do manage to make it through all the 25 infernals spawned by reign of chaos, he should still technically win -

*archimond dies*

...wuhh.....whut.?

Warcraft logic.

plot device

Might have been special wisps.

non sense I opened the last campaign level to see, those wisps only alteration was their movement changed to flying and they where given invulnerability.

Still. There might be some logic behind this. I haven't looked up the Warcraft lore and I forgot the most about it but something feels a-bit special about the spirits(I still know that wisps are spirits) that attacked Archimond as I would remember.

Oh Archimonde why were you a summoned unit with spell immunity in the last chapter, but when the wisps detonated you were just a summoned unit. I guess we will never know...