I would choose the Lore of Flamers for your Slann and try to make sure all your Saurus are equipped with power spears.
Skinks may be useless as the Tyranids are probably immune to bio-toxins.
In other words, good luck! Probably one Carifex could deal with an entire Lizardmen army no sweat, and I hear Tyranids can bring six if they want...
Wait a second, we can bring 6 Stegadons now! Carnifex vs. Stegadon for the win.