I am glad to answer to this thread since I use a lot protoss.
I usually build 2 raf and go straight t2.
Then i build 4 archons and 4 immo. The rest of the army composition depends on enemy strategy.
Archons are great vs bio (good vs mass lings, marines, zealots and mutalisks).
They are also a great tank unit and their short range is perfect because ranged units like stalkers or immortals and colossi can deal extra shots!
Now the answer you are waiting for sylon.
My first thought is that EVERY unit should have a strong counter. Ghosts actually are the best counter you can choose vs archons. Zerg don't have such a powerful counter and this is not balanced.
Ghosts are good vs protoss, anyway, also against immortals. I don't believe that archons should be nerfed or buffed.
I believe that ghost'e emp shock should be fixed once more. 50 dmg is still a lot. 25 sounds better to me.
Can you test if the emp shock is REALLY 50 also vs immortals? (It seems 100 to me, but I may be wrong).
Do you know what I like of this map. The fact that units have real or similar stats to the original sc2 units. I suggest you not to change the stats of the units but to change only their cost or their abilities.
An archon that has something different then the original 350/10 is NO MORE AN ARCHON ^^.
Don't ruin this, please.
So briefly: change emp shock dmg to 25. Low the cost to 300.