The last place I worked had a large HR department extremely capable of dotting "i"s and crossing "t"s. They used this to follow corporate mandates like "cutting the bottom 10%" and so forth. I saw many very good, hard-working, and capable people get pushed out, and many others left in disgust. All technical folks were considered fungible. My experience is that HR will keep the toxic manager as long as they can, and get rid of a long line of subordinates in an attempt to keep the fire under control rather than risk an explosion.
Also, I second the use of contractors if you want to hire and fire as you need. I have worked with contractors, selected contractors, and supervised contractors. My luck has been mostly good, but only because I have only used them for either very well defined projects with a fairly tight boundary, or for projects not in the critical path.
But, contractors are not employees. If you are doing anything core to your business, you take a big risk with a contractor. They take their hard-won knowledge with them, and if someone decides to pay them more, they can drop you in a flash. They can also leak information, even unintentionally. Kind of like a FET leaks current when it's supposed to be off. Sure, you can take legal action, but there goes your savings.
Cheers,
John