HappyDaze, if your adversaries are sufficiently skilled in their attacks, they'll be able to generate enough strain removal with Advantages to keep pace with the PCs. If you're currently having problems now, considering bumping up NPC attack skills.
Often the bad guy goes from 0 Strain to exceeding threshold before they get to take an action. Two or three PCs will usually pounce on one baddie when the initiative allows. They are aware that Strain can be brought down by Advantages, but they also know that someone that can't take and action doesn't generate Advantages. It's more metagaming, but the idea of grouping fire does make sense too, so it's only convenient that there's another metagame perk here.