I have no issues countering them. If they're approaching just shoot once or twice into them and move backwards while hugging the wall and while doing this, continue to feign an engagement, moving forward slightly and just shooting not even at them but rather to -blam!- them and make them think you're gonna stand your ground and keep prefiring at the corner. Once you've pinched them enough, you can actually just slide toward them and gun them down before they can charge up. This works because a good Fusion Rifle user will catch on to what your doing and will try to time their shot to when you peek but you'll never expose yourself fully. An even better Fusion Rifle user simply won't engage and at that point you can keep engaging in your terms, especially if you have a sniper rifle. I went flawless 2 times last week using this strategy. It's even more effective with witherhoard because even if they back out, they'll have to fully commit to looking for cover or get peek finished.