The two defense dice are usually not a big problem, but combined with at least one Evade token (which he gets a lot of the time) and it can be a problem to land even one hit.
Last night I took this list:
Oicunn + Darth Vader + Mara Jade + Ysanne Isard + Expose + Dauntless
Backstabber
Obsidian Squadron Pilot
Obsidian Squadron Pilot
With all due respect this is a big part of your issue right here. You have a mini swarm (and with only 3 TIE's make sure to put plenty of emphasis on the mini part) that all roll just 2 attack dice (yes Backstabber is sometimes an exception) but they are paired with a turret that in actuality isn't that strong of an attacker.
Yes Oicunn can inflict some extra damage but we aren't talking about anything crazy.
One of the big issues is his load out. Expose sounds like a good idea but in practice it's actually inhibiting your attacks if you don't do it right. Yes you throw an extra attack die but it's at the cost of a focus or target lock in which you would want to modify your dice. It also eats up your EPT slot so you can't add Predator or something else.
In my opinion you should tweak your set up on your Decimator (different pilot, crew, EPT, etc) to improve its attack capability and/or drive down the cost.
In addition I would highly recommend ditching the 3 TIE's for something else such as 2 Interceptors, one of which could be Soontir.
TIE fighters are great but you often need enough of them to be able to get enough damage through.