I've been facing erentil kids all day in comp, and I've learned that positioning isn't the easiest thing, especially with how the maps are designed. You're always level with them and the areas they roam are single hallways with little cover. And, there has to be some non special counterplay, otherwise primaries will be useless and it'd just be a bunch of people crouch walking with fusions, or making firing squads with snipers in the back of the map.

Positioning is a critical skill to actually playing comp, if you can't outposition fusions then there is the reason you are dying to them. Don't enter those single hallways, don't peak out of cover if you aren't confident in the kill. Make them come to you. If they refuse to do so, throw a grenade. This is also the reason that special counterplay is fine. While it is preferable to have primary counterplay, it is not necessary, because shotguns can counter snipers by avoiding lanes and getting behind. Snipers can counter shotguns by keeping them at a distance. Fusions are a jack of all trades, but a master of none.