"- Trying to apha-strike just wont work. Fortitude makes it impossible, and Coteaz can seize the initative near 1/3 of those games. This usually means an auto-lose if you have deployed aggressively."
"- "On your face" tactics will also fail. Purifiers can shoot 8 S7 shots from their rhinos, wich combined with Psyflemens, psybolts and strikes means your 2-3 wyches units will fade out quickly, or the remnants won't be strong enough to be a problem."
"- Going second, deploying far away and trying to hide is for me the correct way. Nightshields help here, as well as going full reserves if there's no BLOS elements and the GK player has deployed aggressively."
100% correct. If your opponent makes you go first, still hide like a girl as you have the mobility to pop out on turn 1 should you not get seized.
"- Then, turboboost on one flank (out of psycannon range thanks to nightshields). Better to pick the weakest flank."
"- Prioritize Psyflemen."
Dear god correct.
Pretty much everything you said is right when dealing with ranged GK. Nightshields cut down pretty much all of their firepower if they're using Rhino bunkers. Bit more difficult if they're using Razorbacks as it's more 48" stuff coming your way, but at least you can (usually) shoot the infantry as they won't be inside if they want to shoot. The Psyflemen and Razorbacks generally either need to be your #1 firing priority or #1 hiding-from priority.
The only reason I take Enhanced Aethersails on my Wych Raiders are for pushing flanks to get them into CC with a back-line Psyfleman as soon as possible. That 5 point upgrade works wonders
Another unit to seriously consider are Reavers with Heat Lances. They are nearly twice as effective at destroying vehicles than Blasters and Dark Lances (providing you're in melta range), and the 36" turboboost will again allow you to get near or behind pretty quick. You can also force your opponent to give you his back (of his Psyflemen) by flanking like this - if they turn to shoot the targets then chances are you'll be hitting Av10 with the rest of the army. If they don't turn, then the Reavers will be AP1 meltaing Av10 which is even more devastating.
Rhino/Razorback spam lists generally have quite poor mobility which should be exploited. If the passengers want to fire from inside then he can only move a maximum of 6" with the vehicles per turn, something that providing you have a fairly good amount of terrain on the table you can use to your advantage. Kite him around, block LOS from as many Rhinos as possible and focus on popping them one at a time (again, NS also help greatly with this). Venoms can then do the clean-up work on the passengers.
Not much else to say. Any list with Psy/Riflemen is hard for DE. GK as a whole are quite easy to deal with except for that one unit (which everyone has issues with, both gameplay and otherwise