Gretchin.
That's all.

"Wow, I've nearly killed all of those damn Gretchin... just 100 more and I've tabled you!"
"What? Oh, no, I couldn't fit them all in my deployment zone. I still have another 2000 in reserve. *smiling*"

As many sentinels as I can fit, with a variety of weapons.

Everything dies

5 Monoliths
15 Spyders with Fabricator claws
As many min. size scarab swarms as I can get

Obviously terrible. But I have the hilarious image of a line of monoliths rolling forward all with doors open, Eternity gates sucking up everyone... and every turn a squad of 20 Scarab bases comes scuttling out from behind the armored wall.