Surprise Attack from the Avenger Tree does not work with Assault Rifle and Two Gun Mojo. In the PTS, I did everything i can to NOT have any crit or dex on my test character. I pick Fire Strike, Ice Blast, AR, and 2GM for this testing. Here is my result:
As you can see, Assault Rifle and Two Gun Mojo does NOT benefit from the Surprise Attack spec in the Avenger Tree.