I don't use FL, I use CuBase, but if I understand it right, Slayer is basically a VST softsynth. I don't know what controls you have available, but in general you'd want to mess with the parametric EQ-type controls (to scoop mids or whatever) and the gain. You could also run it through other amp simulators or VST signal processors. I'm not that familiar with what's out there to do this, but if you do a Google seach for VST effects you should find lots of stuff, some free, some not.
How well does it emulate the sound of guitar playing in general?