Yagura, the Sucker Punch point is that the successful strategy for dealing with Shell Smashers is to punish the user (as Expulso did in your match) on the Shell Smash turn, getting them down to a point where priority or a weak attack from a bulky Pokemon can take it out (Slowpoke's Scald does 33% minimum, Psychic does 43%+). Yes, it basically means that Omanyte pretty much gets one guaranteed kill, and if Omanyte gets a crit, it's gg...
Okay, yeah, I think I'm talking myself into this... I'm like 50% there but keep talking.
On the other side, if anyone wants to *keep* Omanyte in the tier, speak now or forever hold your peace.
Also, you forgot Stunky. :p