I should drag myself into writing a story of how humanity as it exists today would ACTUALLY fare against a race of creatures with the power to traverse dimensional barriers, wield arcane arts which violate every law of physics, are truly determined to take over the world, and experienced (by the simple fact that they're conquerors and not happy-go-lucky candy-colored idiots) in tactics of espionage and warfare.
Be honest, we wouldn't stand a chance in hell.
But such a story is... really boring. It'd be as one-sided as REAL aliens showing up and wanting to blow us all up. As I've noted before, they'd just sit in high orbit and fling small asteroids at us. Basic mass-drivers. We'd have no defense whatsoever, and for god's sake, any missiles we'd launch would appear laughably slow to beings with ships which can travel at the very least at light-speed. They'd blast them to bits before they approached within hundreds of miles.
Eh, maybe if I'm really annoyed at humanity one day, I'll slap something together just as a demonstration that humanity is not as invincible and all-knowing as it deludes itself into believing.