There's no such thing as not being able to defeat someone. Iris's golem still has decent mobility, which means it can't be a fortress impervious to attacks. So, it can be taken down. If it was something invincible, Athens would have attacked Krishna using that model only. 20 of them would have got the whole job done. Or 10 of them and a bunch of lesser models to clean things up. Who knows why fools keep telling him it's not possible.
They need to dig up that gun for Rygart to use.