Epi5M1 Spartan Miller is the closest SpOps mission to having infinite enemies. They're Prometheans, and there are 11 waves of them. All waves over 5 have multiple Z-750 Special Application Sniper Rifle wielding enemies, so it gets fairly aggravating later on.
Aside from that, there are no real infinite enemy missions in SpOps. It may seem like that at times (especially when one is trying to do SLND) but no.