One archer is not an issue, but people need to remember, this is a TACTICAL game. Pause the game, and look around. I have played on Nightmare from Day 1, and sure, there are fights where I have to reload and reassess the situation, but once I take the time to really look at the situation, the Battles become a lot easier (By easier, I mean from being literally impossible, to beatable).
One such fight, taking away spoilers. Has you in some dudes Estate, with some girl in a disguise, you turn right and Boom, ambush with about 12 enemies. 8Archers, 2 Sword and Shield guards, a Mage (Yellow) and the Elite Boss, a Girl with a very big sword. This fight was incredibly hard straight up, I came close a few times, but the archers started using Scatter shot IN PERFECT succession to keep my group stunned. I let my arrogance go and pulled the screen back to isometric. Looked around. And saw my plan.
Use Morrigan to cast a Sleep spell on all the Archers. The Mage And Guardsmen would resist, of this I am sure. And they did.
Right after the sleep, I used Leliana to pinning shot the Mage, it landed, as mages have bad Physical resistance.
I then got my entire group to run and take cover in a Library, that completely obliterated the Archers advantage of line-of-sight and range.
The Elite boss followed my Tanker (Alistair currently in the Full Juggernaut Plate armor) He and Leliana's wolf (Second specialization ranger) held him, while I used myself and Morrigan (Between healing spam) to ambush the Archers as they came through. Cone of Cold + Pinpoint strike +Momentum. Let me put it this way
Shattered!
Shattered!
Shattered!
Shattered!
Shattered!
Shattered!
Shattered!
Shattered!
(Literally that fast)
The fight was fairly straightforward from there, I had Alistair kite the Elite boss around the bookcases while everyone focused on her Guardsmen afterwards, it was a simple matter of outlasting her extremly powerful strikes.
The point is, you really need to take the time to assess the situation and deny advantage to your enemies. You COULD just lower the difficulty whenever an archer is on screen, but I have not run into a battle that couldn't, well, be cheaped into victory by basic tactical advantages. The Above fight was easily the hardest I came across though, Not Flemeth, nor the High Dragon was even half as difficult.