He would flick the lice at you and turn it into a oak tree in mid flight
Doesn't work, unless you're getting tricksy with time or fate. Transformation spells, like almost all other spells, are instant actions. Transforming something in mid-flight, after you have thrown it no less, is something that would only be possible for a reflexive spell. For this to work you're going to need to throw on conditional triggers and work with oak trees that have been pre-transformed into lice, then give them a trigger that ends the spell duration upon them being thrown. (That does of course raise the problem of any time you are hit with a supernal dispellation from Prime, dozens of oak trees suddenly form in your head...)

I'd also say... lice aren't aerodynamic. You can't throw them like a dart or flick them like a pellet. You would need to attach them to something that flies true, even if you did have some reflexive transformation spell.