I had a chance to play some off-hours recently and, with fewer guild mates online, found myself soloing a ton. I ransacked both Weapon Shipment and Wrath of Flame in that time. In the latter quest, due to the layout, I was able to attempt quite a few assassinates and double assassinates. I tracked my success rate and will post them here for folks to get an idea of how high a DC you need to be successful.

First, Dresek's Assassinate DC is 38. Not great, but not bad. When in Assassinate mode, his int looks like this:

14 base
2 tome
1 Deadly Shadows
9 ToD ring
----------
26 Total

That gives him an assassinte DC of 10 + 20 + 8 = 38. Note that when I finally re-pull a +3 int tome, I can use an airship buff to gain another bracket.

So with an Assassinate DC of 38, I attempted 28 Assassinates in Wrath. 6 of those were attempted doubles. The result:

8 Success
30 made saves

So a success rate of around 30%.

However, if you count the doubles as two attemps with one success, it's really a success rate of 9/36 = 25%.

Shavarath is probably the hardest area to get an assassinate in -- possibly due to the Shavarath effect -- but one of the places where they are the most helpful. This is why I tend to recommend against people building their character around the Assassin II ability. I only had 8 kills from Assassin II but had 4-8 kills each run from rolling a 20.