Agressive, not violent. Don't go nuclear on it
But also, if those wires have been in for more that 30-60k, they should be changed anyways.
On my plugs and wires, which were original, (95,000 miles), I had to use every last ounce of my full upper body strength to get each and every one off. The back being the worst of course. Just keep twisting and pulling. It also doesn't hurt to bend the boot off the plug at an angle. If you've twisted it and its popped already, then you've gotten the boot to let go, now you're fighting the metal plug clip to release. After hundreds of hot cold cycles, its essentially forged itself on there, so a sideways prying motion usually pops it off if you can locate the axis of the connection point. Works for me.
But of course, pliers are an option.