Sorry I'm late. Thought this was a thread about Druid openings, so it wasn't on my radar.
I play a fair bit of Domination - more than Arena lately.
Sslaks guard everything in the zone... as long as they have a Guard marker. Remember, they only get 1 guard marker each round. Once that Guard marker is gone, anything in the zone EXCEPT THE ORB is free game. Of course, you may need to heed other creautres with Guard markers, Bloodthirsty rules, etc, but basically the Sslak is only on guard until it's been hit or somebody uses Knockdown or Elusive creatures or uses Shrink on the Sslak, Ranged attacks, Zone attacks (very popular in creature-heavy Domination; Firestorm is a popular finisher in our meta), etc. There are lots of ways around a Sslak, as with any guard.
Also, I wouldn't say that turtling is a viable strategy in Domination. Sure, you might have a slightly easier time n not dying in Domination, but generally if you turtle, you also abandon the orbs which makes them easy to steal, and you'll lose on V'tar.