My experience with deer flies is not too bad. I developped a technique. They are so big that you can't miss them when they start hovering around you. I just pretend to ignore it and let it land on my arm or whatever part of exposed skin. I let it settle for a few seconds maybe, and just before it bites me, I swat it diagonally which makes it drop on the ground and then quickly step on it to crush it and finish it mercilessly.

Black flies are another matter. They are so small and are like little piranhas, they gang up on you and when they bite they actually remove flesh. I once ended up to the hospital on account of black flies.

Frankie