I think the Fleshpound solo-kill trick is easy enough to be mastered. If there are no Sirens behind, of course. Only thing you need to learn is distance to make first HSg blast, and how much you need to retreat back to have no damage from your nades. Other manipulations are simple.
Scrake killing technique is more complicated. First you need to make precise hit to the Scrake's head with axe or katana. Second you need very fast switch to HSg, sit down and make 100% precise shot, so all pellets hit the head (if using katana). I personally prefer axe over katana. First, I have much experience of head-hitting Scrakes with axe from playing as zerker (with katana I'll miss the head too often). Second, it's easier to switch to HSg, because it is the only weapon assigned to the key (Axe+Pump Shotgun+Hsg) while with katana you mostly equip AA12 + HSg, and both of them are accessed by the same key (can be used "Switch to last weapon key" too, but it's too complicated to me). Third, axe doing more damage, so HSg blast can be less precise: 17 of 20 pellets must hit the head on 6p HoE, so it requires 85% precision, comparing to 100% with katana.
At the beginning I thought removing flinching ability for non-berserkers will solve the problem. But it isn't so simple. Actually this trick is used very rare due to required skill and fail probability are too high. It is muche easier to stun Scare first, then finish him by support's HSg double blast. Scrake can be stunned by many ways: berserk axe hit to the head, or backstab with axe or katana, Sharpshooter shot with LAR or xbow, LAW rocket, or even support can stun Scrake by himself by axe's alt-fire hit to the head from behind. And in those situations skilled support are excellent Scrake finisher. For example, you need 5 LAR headshots to kill Scrake on 6p HoE. Same amount of hits from berserker alt-fire axe from the front. In the same time support can finish the Scrake after only 1 of those hits. So it makes not Support Specialist to support the team, but team a little bit support Support Specialist to kill Scrakes. Same with Fleshpounds. Other zeds can be killed by Support alone without good skills required.
Doesn't it makes support overpowered? I think it does.