[@Prince of Seraphs]

Decapitation and being riddled with bullets. Being impaled by giant spikes would also kill him. Crushing him with a giant rock might work too, if it's heavy enough.

If you're looking for his kyrptonite, then aside from EMP's I've never made one up because he was never immune to traditional methods in the first place. He's just more capable of handling it then most others. If you can stick a knife between his armor and hit his heart, where that little machine that makes/maintain his aura is at, then that would kill him too. But I'm sure if you stuck a knife into everyone's heart, that'll kill them. It's one of the reasons he wears armor. 

But I'm sure that's exactly what you're asking for. You want the exhaust port to his metaphorical Death Star. But perhaps I'm wrong, so clarify for me, where do you want Gren's weakness to be?