Are you really sure you want a truly even fight? In a truly even fight, it's almost certain one or more PCs will die.
I'd suggest that what you should go for is a challenging fight, with the illusion of being truly even.
There are many ways to do this. The simplest method -- other than simply fudging on the fly, and there's nothing wrong with that -- is upping the Bad Guys' defenses -- AC, saving throws, and so on -- significantly, while lowering their offense significantly. This makes the PCs truly work to defeat them, but makes it very unlikely the PCs will get slaughtered.