Let me guess, he is staying in the back and out maneouvering you as well as spamming Long range Lance weaponry!? I'm quite sure!
I have little experience Vs. DE but I can help you with what I DO know.....
They have superior shooting and shooting range which makes it difficult for us. Heavy Venom Cannons could prove useful because of the range and the new rule set simply puts any part of the blast marker over the hull is a full strength hit, instead of the old rules requiring the hole of the template. You just have to hope that they stay in your firing lanes, so you will need multiple of these.
I used Y-Genestealers to good effect but at an expensive one. They are great at surprising your enemy in the terrain that they hide around, go with the extra attack to help ensure Rending hits. The downside is that they are subsequently left vulnerable but it works. Sometimes it requires sacrificing pawns to acheive another purpose as well as distraction while other units move up.
Flyrants are a win. You can Vector Strike with these as well as use dual TL-Devourers which should cause at least enough glances to their Raiders to reduce their hull points to destroy them.
Regarding Gargoyles, well, I haven't used them with the new rules set because I think they have lost their zing and impact because the new ruleset is so focused on shooting now which means overwatch will destroy them prior to the charge. They can be a great counter-assault unit I am sure of that. Consider charging with a Flyrant and follow up with Gargoyles, that could work.
I would discourage the use of Shrikes, they are expensive and fragile and the Lance weaponry will insta-kill them easily.
Thats all I can think of right now.